Definition of metagame
The "metagame" could be translated as "para-game". Para-game should be seen as a para-language. Para-language is what we communicate to our interlocutor, without using words. If we say "I have some news for you" with a smile or with a sad face, the facial expression will be a meta-language. In poker, meta-language is all the information we have about a player that is not technical information like a tracker can give us. For example, if a player is passive in general, but very aggressive with you, that is metagame. That is information that adds to the decision making. This player is objectively passive, but the metagame (the subjective observations of his play) tells you that he is aggressive with you.
The art of the metagame
Metagaming is an art. To get metagame information about your opponents, you have to be observant. The information obtained from the metagame is the result of our subjective analysis. For example, a player has just 3bet you for the 3rd time in a short time in position. Metagame analysis: we can suspect that this player likes to play you in position. Perhaps he finds you weak when you are out of position? But beware, here, maybe your metagame analysis is not accurate and your opponent has only received 3 excellent hands in a short time.