Cultural Marxism is a conspiracy theory invented in the 90s by American conservatives. They use it to attack all the progressive movements trying to improve America; the theory claims that feminism, environmentalism, social democracy, anti-racism, and anyone who doesn't meet the theorist's conservative values, is part of some crazy conspiracy to undermine american values.
In order, feminism seeks to prevent discrimination based on gender, environmentalism to protect the environment we live in from being destroyed, social democracy seeks to protect the poor, and anti-racism groups seek to protect racial minorities.
If American values are based on 'all men are created equal', then only feminism really contradicts American values, but i believe most Americans would be happy to amend that as meaning 'all people are created equal'.
These progressive movements have their roots in the enlightenment, the French revolution and the American revolution. These ideas and philosophies (of preventing discrimination against women, the poor/lower classes and racial minorities) came together in europe in 1848 in sporadic uprisings an rebellions across the continent. All of which were suppressed.
Marx then wrote about having the workers of the world come together - and he meant industrial workers, no under-educated agricultural workers, infact he explicitly said Socialism wouldn't work in Russia. Various communist groups failed to take hold in Europe, and their ideas were co-opted by democratic groups, ones which eventually united with women demanding the right to vote, with labour unions dedicated to improving working conditions for the poor/working class (slavery was abolished much sooner than in the US).
Social Democrats and Democratic Socialists both deviated greatly from Marx and continue to be a part of western culture. These ideals are not at all antithetical to USAian values. They believe in our shared humanity and our shared environment, they believe in Human Rights (as promoted by noted American Eleanor Roosevelt, you may have heard of her). Feminism as a movement has gone deep into inclusion, by studying academically the intersection of different oppressive forces, how being a poor black woman may be a different experience from being a poor trans woman, for example. And so, to my knowledge, is the only philosophy which seeks to unite all oppressed groups in the language they use to oppose their oppression.
There 90s conservative reactionaries coming up with a new conspiracy theory to brand all their opponents as 'enemies of true American culture' are very much responsible for the partisanship and division within American.
I know of no-one, outside of this webforum, who seriously subscribes to this lunatic conspiracy. Who believes the equality for women, for indegnous peoples, and for racial minorities is in any way contrary to American values; the same goes for people believing that polluting our environment is anti-American (though there are more who would debate whether Carbon Dioxide is a pollutant, they still believe we should protect our shared environment).
Tinfoil lunacy, at its best.