There's two definitions, the former means redfash who overlook the fascist elements of former socialisms, particularly those of Marxist-Leninist orientation. (the fact they use tankie rather than redfash should tell you they're not entirely comfortable using fascist as a pejorative, want to guess why?)

The latter is basically a dog whistle used by reactionaries to mean communist. Basically a synonym of "woke" for those not overtly racist and applies to anyone with left-of-hitler politics. Think your blue maga types who swear they're progressive.

Regardless, the word was invented by the CIA to sew division among the left and should not be used by anyone.