IDK, I've seen people use that phrase with at least 3 apparent distinct meanings here, so I'd guess it's just a buzzword for them to signify something "bad" the "left" is doing
It's a useless term. By accusing the "other" side of virtue signalling you're tacitly implying that it's a bad thing, while at the same time being blissfully unaware that the action of doing so is virtue signalling to "their" side, which must also be a bad thing.