Tickets are a special currency used to enter Special Events, which are open every weekend.
To enter on these events you can spend 1, 5, 10 or 20 tickets. The rewards you get from the event is multiplied by the number of tickets you used to enter.
There is a 25% chance of obtain Tickets on Brawl Boxes. They drop in sets of 1, 2 and 3.
Tickets can also be bought in the Shop while an event is open.