What is metagame?

A metagame for any game is the way that the game is played. Knowing the metagame is not just knowing the mechanics of the game, but knowing what to expect your opponent to do. It is the relative frequency of each possible tactic, maneuver, build, etc.

One of the easiest to understand examples of this is with collectible card games like Magic: The Gathering and Hearthstone: Heroes of Warcraft. Knowing the metagame in a collectible card game is knowing what styles of decks you might come up against. For example in Magic if your opponent plays an Island on turn one, are they attempting to lock you down, draw you out, make evasive creatures, or any variety of things blue Magic players can do? You don't know if you don't know the metagame.

In StarCraft, for a more localized example, Protoss should wall off against Zerg, but not against Protoss or Terran. Zerg might 6-pool, and if you don't wall off then your opponent is probably going to beat you. As Protoss continues to wall off and 6-pools continue to be ineffective, Zerg players might stop using that strategy. As Zerg players stop 6-pooling, Protoss players might not wall off as often, making the 6-pool effective again. You have to know the current mindset of other players to make the optimal play. That is the metagame.

The metagame is always evolving for any game, so you have to keep playing or reading about your game to keep up with the current metagame.

I like the wikipedia definition: "In simple terms, using out-of-game information, or resources, to affect one's in-game decisions." So, almost any game could have a meta-game.

In addition to what Strix said, metagame (in a more localized sense) can also be influenced by knowledge of the other players. Perhaps you weren't able to scout someone, but you have watched ten of their replays, and in eight of them, the player used the same strategy. You can prepare to counter that strategy even though you haven't actually gotten any useful info in the current game.