Wikipedia has a decent summary:

A 21st-century conspiracy theory regards the Frankfurt School as the origin of a contemporary movement in the political left to subvert traditional western cultural norms, referred to as "Cultural Marxism" by theory proponents. It advocates the idea that multiculturalism and political correctness are products of critical theory, which originated with the Frankfurt School. The theory is associated with American conservative thinkers such as William Lind, Pat Buchanan and Paul Weyrich, and has received institutional support from the Free Congress Foundation.

According to Chip Berlet, who specializes in the study of extreme right-wing movements, the Cultural Marxism conspiracy theory found fertile ground with the development of the Tea Party movement in 2009, with contributions published in the American Thinker and WorldNetDaily highlighted by some Tea Party websites.

More recently, the Norwegian terrorist Anders Behring Breivik included the term in his self-authored document "2083: A European Declaration of Independence", which along with the Free Congress Foundation's "Political Correctness: A Short History of an Ideology" was e-mailed to 1,003 addresses about 90 minutes before the 2011 bomb blast in Oslo for which Breivik was responsible.

Philosopher and political science lecturer Jérôme Jamin has stated "Next to the global dimension of the Cultural Marxism conspiracy theory, there is its innovative and original dimension, which lets its authors avoid racist discourses and pretend to be defenders of democracy."