Arena is a game mode in Plants vs. Zombies 3 where you can compete with other players to get seeds and gems and get promoted to new advanced leagues.
The Arena is divided into different leagues, with each rank having a total of 25 players. To earn trophies, you need to spend some boxing gloves and beat the opponent. The more trophies you have in Arena the more likely you are to receive higher rewards and be promoted to another rank (after a week). However, better/more rewards are given in advanced leagues.
List of Ranks