Since the opponent's class is not revealed until the match has started, and the contents of their deck usually does not become clear until some way into the match, being able to predict the opponent's choices offers a strong tactical advantage. The meta is primarily of interest to players seeking to anticipate the choices of their opponents.
The metagame or meta describes the trends of deck and class choices currently seen in Hearthstone. If you know what decks players are bringing, your deck can be especially good against 'the meta'.' - Ben Brode
'The metagame is the mix of different decks that players bring onto the ladder, that you can expect to see.