It is a monthly competition where players compete against each other based on total trophies i.e. number of arenas owned by them at the end of each month with some rewards given out to winners like rare jewels, rainbow souls, etc. If you clear all levels in arena mode (which has a total of 100 levels) you will get a reward for your dedication. The player with the higher trophy count wins the battle and their trophy count decreases while the opponent's increases in proportion to the difference between them.