There’s two definitions, the former means redfash who gravitate towards 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.

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