These are called Showdown Tickets and can be used to enter Showdown Tournaments. You can compete in 100-player matches clearing rounds, and the last player standing gets the Jackpot.

Each Showdown event will last only a few hours, but you can enter as many matches as you want. You get a free match entry for every event, and after that, you can use Showdown Tickets to enter.

When a new event is available, it will be accessible in the More Games tab or by clicking on the card that appears in the Home Lobby.