Cultural Marxism generally refers to one of two things:
First — extremely rarely in popular discourse — "cultural Marxism" (lower C, upper M) refers to a strain of critique of popular culture by the Frankfurt School, framing such culture as being imposed by a capitalist culture industry and consumed passively by the masses.
Second — in common usage in the wild — "Cultural Marxism" (both uppercase) is a common snarl word used to paint anyone with progressive tendencies as a secret Communist. The term alludes to a conspiracy theory in which sinister left-wingers have infiltrated media, academia, and science, and are engaged in a decades-long plot to undermine Western culture. Some variants of the conspiracy allege that basically all of modern social liberalism is, in fact, a Communist front group.
This conspiracy theory hinges on the idea that the Frankfurt School wasn't just an arcane strain of academic criticism.[note 1] Instead, the Frankfurt School was behind an ongoing Marxist plot to destroy the capitalist West from within, spreading its tentacles throughout academia and indoctrinating students to hate patriotism and freedom. Thus, rock'n'roll, 1960s counterculture, the civil rights movement, the anti-war movement, homosexuality,[3] modern feminism, and, in general, all the "decay" in the West since the 1950s are allegedly products of the Frankfurt School.[4] It's also the work of the Jews.[5][6]
The conspiracist usage originated in Nazi Germany, where Kulturbolschewismus ("Cultural Bolshevism") was used to abuse political opponents. In particular, Jews purportedly were secretly orchestrating the spread of Communism (Jewish BolshevismWikipedia's W.svg) as well as promoting sexual & gender permissiveness ("sexual Bolshevism").[7]
If anyone rants about "Cultural Marxists taking over culture!", feel free to remind them that they're spouting literal Nazi propaganda updated for the modern era.
https://rationalwiki.org/wiki/Cultural_Marxism