Here's an article that might help explain a bit on it. It's from the Communist Party of Ireland and it's on Tailism and Economism and directly touches on what Lenin talks about

So in wider context, Lenin's talking about what he's seen in the Russian socialist movement at the time (it's prior to the outbreak of ww1 and the schism between the class traitors and the communists).

On the first question: The pandering to the reactionary portions of the working class (tailing) tends to be a mix of economism and divisionist tactics, that is to say they are using the fact that class struggle is the primary contradiction and all else is adjacent contradictions to it as an excuse to outright dismiss and/or deride said adjacent contradictions in order to focus exclusively on class issues. On how it is economism Lenin says the following,

“The Economists [limit] the tasks of the working class to an economic struggle for higher wages and better working conditions, etc., asserting that the political struggle [is] the business of the liberal bourgeoisie.”

In the article from the CP Ireland it mentions that To give a clear example of the limits of the politics of economism, we can recall how women’s participation in the labour market was initially opposed within the trade union movement, on the grounds that it was a means used by capitalists for lowering wages for workers more generally. Other historical examples are how during the time of Z Foster when trade unions that were majority white working class men would go on strike the bourgeoise would hire black strikebreakers to replace them, as black americans at the time were commonly former slaves or children of former slaves whom lived in abject poverty due to the fact that many freed slaves were simply emancipated with no education, training, or wealth and had to struggle to accumulate wealth to increase their living standards, and instead of extending a hand of solidarity to the black strikebreakers and offering to fight for them to join them as coworkers in the union if they stopped strikebreaking and joined them in the strike they instead would attack the strikebreakers and denounce their bosses attempt to replace them with a cheaper and inferior workforce. A Communist would see such events as opportunities to further strengthen the union movement by further expanding the power of the union through the inclusion of all races and agitate among the union workers to educate them on the material benefits of such actions as they're able to. A tailist would see such events as attacks on the working class by the bourgeoise and their lackeys and agitate among the union workers to reinforce union solidarity by undermining proletarian solidarity. A modern example could be the fight against any attempt to paying "illegal" workers minimum wage and extending labor rights and protections to them, or how a workplace union would self-sabotage because of the belief of some workers that their skills are superior to other workers therefore instead of fighting for a collective increase of benefits instead fight to undermine the collective for their own material benefit, or the portrayal of starbucks workers as not being workers as a form of dividing the working class along arbitrary lines against itself.

On the nazi bit, lets say that nazi plumber's in a plumber's union. That plumber's just one voting member. They may try to exert influence within the union on other members with their dogshit ideas, but their dogshit ideas loose against the direct material interests and financial benefits that the union wins. A strong and militant union that consistently fights for it's members will leave these kinds of dipshits in the dust. They'll either settle down because their material needs are met and can hopefully be integrated into the wider pro-worker culture they're a part of or simply shut the fuck up because they know they're in the overwhelming minority.

On the question of terrorism, simply put, Lenin and the communists rejects political terrorism and the "propaganda of the deed" as listed in the article titled "Revolutionary Adventurism". Let me the section that directly answers your question. Do read the whole thing as it's an interesting reading from someone who's brother was executed as a proponent of the propaganda of the deed. Also you can really see some of the character of Lenin come out in how he quote's the Left SR's writings and then immediately in brackets dunks on them. I personally interpret every time a [!] comes up as him going :jesse-wtf:

The Social-Democrats will always warn against adventurism and ruthlessly expose illusions which inevitably end in complete disappointment. We must bear in mind that a revolutionary party is worthy of its name only when it quides [sic.] in deed the movement of a revolutionary class. We must bear in mind that any popular movement assumes an infinite variety of forms, is constantly developing new forms and discarding the old, and effecting modifications or new combinations of old and new forms. It is our duty to participate actively in this process of working out means and methods of struggle. When the students’ movement became sharper, we began to call on the workers to come to the aid of the students (Iskra, No. 2[See present edition, Vol. 4, pp. 414-19.—Ed.]) without taking it upon our selves to forecast the forms of the demonstrations, without promising that they would result in an immediate transference of strength, in lighting up the mind, or a special elusiveness. When the demonstrations became consolidated, we began to call for their organisation and for the arming of the masses, and put forward the task of preparing a popular uprising. Without in the least denying violence and terrorism in principle, we demanded work for the preparation of such forms of violence as were calculated to bring about the direct participation of the masses and which guaranteed that participation. We do not close our eyes to the difficulties of this task, but will work at it steadfastly and persistently, undeterred by the objections that this is a matter of the “vague and distant future.” Yes, gentlemen, we stand for future and not only past forms of the movement. We give preference to long and arduous work on what promises a future rather than to an “easy” repetition of what has been condemned by the past. We shall always expose people who in word war against hackneyed dogmas and in practice hold exclusively to such moth-eaten and harmful commonplaces as the theory of the transference of strength, the difference between big work and petty work and, of course, the theory of single combat. “Just as in the days of yore the peoples’ battles were fought out by their leaders in single combat, so now the terrorists will win Russia’s freedom in single combat with the autocracy,” the April 3 leaflet concludes. The mere reprinting of such sentences provides their refutation.

Anyone who really carries on his revolutionary work in conjunction with the class struggle of the proletariat very well knows, sees and feels what vast numbers of immediate and direct demands of the proletariat (and of the sections of the people capable of supporting the latter) remain unsatisfied. He knows that in very many places, throughout vast areas, the working people are literally straining to go into action, and that their ardour runs to waste because of the scarcity of literature and leadership, the lack of forces and means in the revolutionary organisations. And we find ourselves—we see that we find our selves—in the same old vicious circle that has so long hemmed in the Russian revolution like an omen of evil. On the one hand, the revolutionary ardour of the insufficiently enlightened and unorganised crowd runs to waste. On the other hand, shots fired by the “elusive individuals” who are losing faith in the possibility of marching in formation and working hand in hand with the masses also end in smoke.

But things can still be put to rights, comrades! Loss of faith in a real cause is the rare exception rather than the rule. The urge to commit terrorist acts is a passing mood. Then let the Social-Democrats close their ranks, and we shall fuse the militant organisation of revolutionaries and the mass heroism of the Russian proletariat into a single whole!