League rating system is what the matchmaking is based on in War Robots: it matches up players of similar skill according to their rating. There are nine leagues, most of which are divided into three subleagues.
The higher the player's rating, the more prestigious their league. The system estimates how good Commanders perform in battles and adjusts their rating accordingly. If you win more often than you lose, you will advance faster. Remember though that while higher leagues give players more rewards, they also make it more challenging to stay there.