The concept of "the meta" arises from the idea of players playing a metagame in which they're picking strategies which work well against the strategies other players pick. The idea I think was that it wasn't necessarily the best strategy, but it was one that reliably worked against those strategies others pick, so it highlights the possibility that there are unexplored strategies.

But because it identified popular strategies it became used just to mean that even in single player games where there is no metagame at all.