The Ace Pilot League shows how good you are against other players. The more you win, the higher is your rank in the league and, when the league is over, you receive better rewards.
The player's position on the leaderboard depends on his/her score in the current season. The leaderboard will be refreshed before a new season starts.
To go up in rank you need to win in the league. This will give you better rewards when the league is over and as you go up in rank, allow you to use better planes.
Please bear in mind you can only fly planes your rank allows you to.
This makes it fair for everyone, as matchmaking in lower ranks will not be as uneven as if players could use high levels planes right away. Make sure you press the Rewards button before playing in the League so that you can understand the Tiers of planes you can use in such battles.
With irrevocable ranks, you won't be demoted even if you lose all the stars.
Remember that when a new League starts, Ranks will always be reset to all players.