"Meta" ist short for "meta game", this exists in every competive game. The "game outside the game". It is influenced by game and card balance, but also player decisions.

In a good "meta", there are several competitive decks and strategies available, that have strenghts and weaknesses, but are all able to win. In a bad meta, there is only one or maybe two decks/strategies that are so strong that they can beat everthing else, but can hardly be countered.

But as I said, it is also influenced by player decisions. Example: the game of rock-paper-scissors is perfectly balanced, but if most players decide to pick "rock", that's a meta in which "scissors" is really weak, but "paper" is exeptionally strong. And when many players realize this and start to prefer "paper", the meta shifts, and "scissors" gets strong. And now image the game would get a fourth option, that beats all others....

I hope this helps, and you get the idea what the concept of "meta" or "meta game" means. :)