Your League is a measure of your overall success in a given week. You start in the lowest League. If you do well, your League ranking will increase. If you don't do so well, you'll stay in the same League, or if you're higher up, you may drop down League.
Win games and collect Coins for winning! The number of Coins you win in a week, compared to other players in your League, determines your placement! Higher Leagues earn more Player Cards when they open Card Packs!