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