Meta (or metagame) is an informal game term. The meta is what sort of strategy you expect your opponents to be playing, either because the strategy is strong or because it is popular. Those strategies are often referred to as "part of the meta". Metagaming as a verb means playing against how you expect your opponent to play, both in a broad strategic sense and how you expect them to play, turn to turn.
Each meta is contextual: while the cards may be the same, there's a wide gap between the first official Decipher tournament held in Movie Block and playing a Movie Block informal match on GEMP in 2021.