can someone please explain what is a metagame?

Metagame is the set of strategies that are prevalent at whatever game you're playings, it can be used to make good judgements without exactly knowing a player's tendencies, but assuming he is alike to the average player in the game.

Ex. People who buy in short stacked (20bbs) are typically 3betting light and being as annoying as possible. The metagame of 25NL to 400NL varies in the average aggressions, at 400NL people are 3bet,4bet bluffing and squeezing 4betting cold in the blinds with weak holdings, while at the lower limits typically everyone is a nit because you can be a nit and be profitable at the lower stakes.