Allow this oldfag to give it to you straight, since thankfully this is one term that nobody needs a longpost to explain:

It's the belief that existing cultures are responsible for sustaining all of society's inequalities.
It differs from conventional Marxism in that conventional Marxism argues that economic classes are responsible for said inequalities.

This belief structure is one of the sacred cows of the regressive left, alongside postmodernist theory, and suffice to say there is a big fucking drive to keep the waters on the subject as muddy as humanly possible. Many regressive leftists like arguing it's nothing more than a conspiracy theory, and with good reason - just looking at that bolded text, any Kiwi can immediately recognize that damned near every bit of internet outrage that's happened over the last five fucking years, from the Ghostbusters 2016 controversy to the fucking Autism Holy War itself have, as the core reason they even fucking happened in the first place, that core fucking point in common with one another.