Leagues are a feature that allows you to compete in a PvP Arena with other Epic players around the world. By winning fights and completing daily objectives you collect ranking points that are used to calculate your rank in the league.
There are a total of 6 tiers of leagues, ranging from wood to diamond. All new players start from the wood league but by collecting lots of points and ending up in the top 3 of your league by the end of the week, you advance into the next league level! By reaching the higher and more difficult leagues you improve your chances of earning better rewards. However, if you end up in the lower ranks of your current league or don't play at all, you might be dropped to a lower league.
After 12 league weeks (3 months) the PvP season will end. At the end of the season, additional season rewards will be distributed according to the league and rank you have reached. After that a new PvP season starts and all league ranks are reset.