The first thing it is, is a series of denials. It's the philosophical equivalent of evasive language. The weezle words of economic theory.

That aside, at the core, it's the very stupid idea that economic value is a zero sum game that can be derived exclusively from the Labor effort that was put in to that economic output.

If you take that basic flawed premise, you arrive at every other half baked marxist idea, up to, and including CRT, communism, 'privilege' and ultimately genocide, and starvation at the hands of "seizing the means of production".

If Labor creates value then higher economic reward for equivalent value = exploitation. If Labor creates value then races that occupy higher economic brackets can be considered "privileged" (Marx identified Jews. Modern Marxists identity Jews, Asians and White people). If Labor creates value, then the proletariat own the means of production, and must "seize it back". If Labor creates value then violence is justified. If Labor creates value then violent revolution is inevitable. If Labor creates value, then society just has to a-"woke"-en to class consciousness. If Labor creates value then equality of outcome is the only measure of success, and equality of opportunity is a failure. If Labor creates value then the most important aspect of anyone's existence is their identity, as their economic contribution can be considered moot. If Labor creates value, then entrepreneurship is exploitative, and employees are victims. If Labor creates value, then economic failure is not cultural, or based on individual choices, but MUST be based on systemic oppression. If Labor creates value then this concept of systemic oppression is the LARGEST factor contributing to people's economic position, and therefore must be taught as such in schools.

If Labor creates value, then you can look at anything of value and say "this is ours", milk that value that was created not by Labor but by innovation, and wring it dry until the ultimate collapse of the economy and the starvation of the people. If Labor creates value then it's appropriate to commit genocide against whichever races happen to occupy the higher economic classes of society at any given time. If Labor creates value then the ends justify the means, and a "dictatorship of the proletariat" installed via "violent overthrow" and "despotic inroads" is entirely justified. (Marx)

So what is Marxism? The asinine belief that the act of performing labor is the definitive measure of economic value. It's an assertion of ownership based upon economic illiteracy, and the rest of the ideology flows on from there.

Is it more complex than that? Well yes, it's a set of flakey obfuscations built upon that central tenet, because they know that if you're allowed to attack the fundamental brick upon which every other part is built, the whole dopey ideology collapses.