Seasons are periods during which the players participating in online Battle Arena are ranked against each other on the Leaderboards.
With the beginning of the first Season, all the players have their trophies reset to 0 and are awarded a corresponding number of gems. This way all players can enter Seasonal leaderboards with equal chances.
Once a Season is over, the accumulated player’s trophies will be halved and, again, the player will be rewarded with gems, based on the League they’ve managed to reach.