Tournaments are limited-time competitions with other players that last several days.
In most cases, you need to meet a certain initial requirement to take part in a tournament (each tournament has its own requirement). Any player who meets this requirement may participate in the tournament.
During a tournament, you compete with other players for the top spots on the leaderboards. Those who finish in the top spots receive rewards. The most active players move on to the next league, while the least active players fall to a lower league.