It basically means trend. It’s sorta evolved from the concept of “metagaming” where you’re not just playing the game, you’re gaming the game. People now use “the meta” to refer to the collection of viable strategies for a game, and “the current meta” to refer to what is popular at this moment. This could be types of decks in a card game, character builds in an MMO or a MOBA, or other things like that. Presumably, for twitch, “the meta” is referring to not strategies of playing games, but strategies of gaming twitch. In other words, strategies to maximize viewership and income, and specifically, what is working at any given time.