Stalin is saying that social democracy is the response of the capitalists to a stronger proletariat, and the end outcome of social democracy remains the same as fascism - capitalists remain in power - and will withdraw the treats as soon as the revolutionary desires of the masses have settled down.

Stalin is writing in the interwar period before fascism has really demonstrated certain aspects that we associate with fascism today

extended quote with link to original source material

Some people think that the bourgeoisie adopted “pacifism” and “democracy” not because it was compelled to do so, but voluntarily, of its own free choice, so to speak. And it is assumed that, having defeated the working class in decisive battles (Italy, Germany), the bourgeoisie felt that it was the victor and could now afford to adopt “democracy.” In other words, while the decisive battles were in progress, the bourgeoisie needed a fighting organisation, needed fascism; but now that the proletariat is defeated, the bourgeoisie no longer needs fascism and can afford to use “democracy” instead, as a better method of consolidating its victory. Hence, the conclusion is drawn that, the rule of the bourgeoisie has become consolidated, that the “era of pacifism” will be a prolonged one, and that the revolution in Europe has been pigeonholed.

This assumption is absolutely wrong.

Firstly, it is not true that fascism is only the fighting organisation of the bourgeoisie. Fascism is not only a military-technical category. Fascism is the bourgeoisie’s fighting organisation that relies on the active support of Social-Democracy. Social-Democracy is objectively the moderate wing of fascism. There is no ground for assuming that the fighting organisation of the bourgeoisie can achieve decisive successes in battles, or in governing the country, without the active support of Social-Democracy. There is just as little ground for thinking that Social-Democracy can achieve decisive successes in battles, or in governing the country, without the active support of the fighting organisation of the bourgeoisie. These organisations do not negate, but supplement each other. They are not antipodes, they are twins. Fascism is an informal political bloc of these two chief organisations; a bloc, which arose in the circumstances of the post-war crisis of imperialism, and which is intended for combating the proletarian revolution. The bourgeoisie cannot retain power without such a bloc. It would therefore be a mistake to think that “pacifism” signifies the liquidation of fascism. In the present situation, “pacifism” is the strengthening of fascism with its moderate, Social-Democratic wing pushed into the forefront.

Secondly, it is not true that the decisive battles have already been fought, that the proletariat was defeated in these battles, and that bourgeois rule has been consolidated as a consequence. There have been no decisive battles as yet, if only for the reason that there have not been any mass, genuinely Bolshevik parties, capable of leading the proletariat to dictatorship. Without such parties, decisive battles for dictatorship are impossible under the conditions of imperialism. The decisive battles in the West still lie ahead. There have been only the first serious attacks, which were repulsed by the bourgeoisie; the first serious trial of strength, which showed that the proletariat is not yet strong enough to overthrow the bourgeoisie, but that the bourgeoisie is already unable to discount the proletariat. And precisely because the bourgeoisie is already unable to force the working class to its knees, it was compelled to renounce frontal attacks, to make a detour, to agree to a compromise, to resort to “democratic pacifism.”

Lastly, it is also not true that “pacifism” is a sign of the strength and not of the weakness of the bourgeoisie, that “pacifism” should result in consolidating the power of the bourgeoisie and in postponing the revolution for an indefinite period. Present-day pacifism signifies the advent to power, direct or indirect, of the parties of the Second International. But what does the advent to power of the parties of the Second International mean? It means their inevitable self-exposure as lackeys of imperialism, as traitors to the proletariat, for the governmental activity of these parties can have only one result: their political bankruptcy, the growth of contradictions within these parties, their disintegration, their decay. But the disintegration of these parties will inevitably lead to the disintegration of the rule of the bourgeoisie, for the parties of the Second International are props of imperialism. Would the bourgeoisie have undertaken this risky experiment with pacifism if it had not been compelled to do so; would it have done so of its own free will? Of course, not! This is the second time that the bourgeoisie is undertaking the experiment with pacifism since the end of the imperialist war. The first experiment was made immediately after the war, when it seemed that revolution was knocking at the door. The second experiment is being undertaken now, after Poincaré’s and Curzon’s risky experiments. Who would dare deny that imperialism will have to pay dearly for this swinging of the bourgeoisie from pacifism to rabid imperialism and back again, that this is pushing vast masses of workers out of their habitual philistine rut, that it is drawing the most backward sections of the proletariat into politics and is helping to revolutionise them? Of course, “democratic pacifism” is not yet the Kerensky regime, for the Kerensky regime implies dual power, the collapse of bourgeois power and the coming into being of the foundations of proletarian power. But, there can scarcely be any doubt that pacifism signifies the immense awakening of the masses, the fact that the masses are being drawn into politics; that pacifism is shaking bourgeois rule and preparing the ground for revolutionary upheavals. And precisely for this reason pacifism is bound to lead not to the strengthening, but to the weakening of bourgeois rule, not to the postponement of the revolution for an indefinite period, but to its acceleration.

It does not, of course, follow that pacifism is not a serious danger to the revolution. Pacifism serves to sap the foundations of bourgeois rule, it is creating favourable conditions for the revolution; but it can have these results only against the will of the “pacifists” and “democrats” themselves, only if the Communist Parties vigorously expose the imperialist and counter-revolutionary nature of the pacifist-democratic rule of Herriot and MacDonald. As for what the pacifists and democrats want, as for the policy of the imperialists, they have only one aim in resorting to pacifism: to dupe the masses with high-sounding phrases about peace in order to prepare for a new war; to dazzle the masses with the brilliance of “democracy” in order to consolidate the dictatorship of the bourgeoisie; to stun the masses with clamour about the “sovereign” rights of nations and states in order the more successfully to prepare for intervention in China, for slaughter in Afghanistan and in the Sudan, for the dismemberment of Persia; to fool the masses with highfaluting talk about “friendly” relations with the Soviet Union, about various “treaties” with the Soviet government, in order to establish still closer relations with the counter-revolutionary conspirators who have been kicked out of Russia, with the aim of bandit operations in Byelorussia, the Ukraine and Georgia. The bourgeoisie needs pacifism as a camouflage. This camouflage constitutes the chief danger of pacifism. Whether the bourgeoisie will succeed in its aim of fooling the people depends upon the vigour with which the Communist Parties in the West and in the East expose the bourgeoisie, upon their ability to tear the mask from the imperialists in pacifist clothing. There is no doubt that events and practice will work in favour of the Communists in this respect by exposing the discrepancy between the pacifist words and the imperialist deeds of the democratic servitors of capital. It is the duty of the Communists to keep pace with events and ruthlessly to expose every step, every act of service to imperialism and betrayal of the proletariat committed by the parties of the Second International.

from Concerning the International Situation (1924) by J.V. Stalin

He's saying that Social Democracy and Fascism form the left and right fists of the bourgeois imperialist states. They don't oppose each other from the perspective of those outside the empire and facing it's attacks, they support and regulate each other. Without Social Democracy regulating and sustaining capitalism and mitigating its crises the economic system would collapse and internal crisis would dominate and the empire would be unable to sustain outward pressure. Without Fascism, a social democratic Liberal state would transition into socialism and go too far - so it keeps fascism around to keep the socialists in check. Fascism is the necessary violence of the capitalist state, even a social democratic state, to sustain itself and enforce the limits of democracy (ie, to not allow the poor to vote away the wealth of the rich. There must be a hardline protection against that).

From the perspective of those outside of "Social Democratic Liberal Democracies" facing their imperialism, all they are seeing is fascism being imported into their country and invasions and meddling from fascist attacks. The social democrats keep things stable at home so that the fascist imperialists can extract value from the colonies and beat down any leftists inside the core. The fascists brutally destroy communists and leftists on behalf of capital, while being fed and clothed by the social democrats. It's a symbiotic marriage.

Keep in mind that fascism is the most powerful tool of anti-socialism. If the masses can't be placated, fascism must be deployed to save capitalism. So usually the quote is used when social democrats go mask off like with sdp and the freikorps. Basically Social democrats, like other capitalists have to rely on fascism as a safety net.