It's from video games. People started using it because you might have an optimal strategy but then people started using strategies to counter the optimal strategy. A meta strategy. It makes sense when you have PvP but it's also used to indicate general strategy now. Meta might change if the game changes or if someone figures out a more optimal strategy. So it means "the currently perceived optimal strategy, taking into account other strategies".

It's a shortening of metagame

I'll try my best but it'll probably be wrongish.

It usually relates to a notable thing happening in a closed community or specific subject.

There was PM spam from "the fediverse chick" a while ago, and that became the Lemmy meta for a bit.