From Grammarly (which can explain it much better than I can 😅) —
"In the world of gaming, meta is used in two ways. Meta can be used as an acronym for “most effective tactics available,” and calling something “meta” means that it’s an effective way to achieve the goal of the game, whether it’s to beat other players or beat the game itself. Meta can also be short for metagame, which is using information about the game, derived from the world beyond the game or its rules, to influence the outcome of the game or gain a competitive edge."
Kindly note that when you're playing competitive games, "meta" usually refers to the second definition and is used as a weak insult. :)