Cultural Marxism (also Cultural Bolshevism or Kulturbolschewismus, in the original German and frequently what the alt-right is talking about when they say "The System") is a conspiracy theory in which the Frankfurt School (a body of Marxist writings) and critical theory (critiques of society, with some origins in Marx's work) are the products of leftists' concerted efforts to (successfully) sneak communism into academia and culture at large. In turn, the conspiracy goes, modern gender/race/sexuality/etc. movements are ultimately grounded in these communist writings — and so they are communist themselves. The theory is absurd. The Frankfurt School was obscure and had a negligible impact on broader society. The methods used by social justice advocates — analyzing society through lenses of class/gender/race/etc. — long precede the Frankfurt school.
Most often the examples of "Cultural Marxism" will be from the so-called Science Wars a period when a bunch of postmodernist critics who really should have known better tried using critical theory on demonstrably accurate scientific principles as if they were social constructs, leading to some truly bizarre claims such as Sandra Harding describing Newton's Principia Mathematica as a "rape manual" or Luce Irigaray's claim that E=mc2 is a "sexed equation" that "privileges the speed of light over other speeds that are vitally necessary to us." While this is a thing that definitely existed and still does to an extent, it is not and never was the all-encompassing conspiracy it is painted as, and many of those involved in producing the examples cited have stated they regret their prior work and acknowledge all they did was gave ammunition to reactionaries.
In short: social liberalism is actually communism in disguise, and the leftists know it. (Nothing new here!) Calling something Cultural Marxism is an easy way to dismiss it without actually engaging with its arguments.
People who whine about Cultural Marxism often attribute it to a supposed International Jewish Conspiracy.
- Sandra Harding, The Science Question in Feminism, p. 116
- Postmodernism Disrobed, Richard Dawkins, Published in Nature, 9 July 1998, vol. 394, pp. 141-143