The so-called meta in gaming has become an important term in e-sports and gaming culture, and means different things depending on which game you play.
What does meta mean? Meta comes from Greek and means "after","beyond" or "transcending". In common usage, the word meta often refers to something higher-level in a category. For example, metadata is data about data. A meta-joke is a joke about jokes.
The meta-game in games like Starcraft means just that. The "game within the game", playing psychological games with your opponent, setting traps, provoking, changing levels. According to this interpretation, the metagame is not about the best weapon or the best hero.
However, the meaning of meta in gaming can be very different. Much more common is the term as a description for strong weapons, classes or strategies. This can be, for example, a champion in League Of Legends or an assault rifle in Warzone that everyone is playing. Here, meta means an item or character that is currently the fastest and most popular way to win.
Games that regularly get new content and are relatively competitive are especially susceptible to metas, as the balance of power is always changing. Often things that are meta are simultaneously "OP", meaning "overpowered".
Partly because the balance was not easy to assess before, but partly probably also to make the new (often paid) items and content a bit more attractive. In such cases, elements are often made weaker again ("nerfed") or others strengthened ("buffed") over time to restore the balance.