This is a time-limited event.
Work together with your teammates to win levels and progress through the reward tiers. You will be able to see the number of levels the team will need to win under each reward chest.

Complete as many levels during the event to progress towards each reward chest target. 
The top contributor in the team will also earn an extra reward.
If you leave and join a new team during the event your progress will not count for that event and you will need to wait for the next event to start.