Once you have passed all the regular levels in the game, you will gain access to the Gaia's Challenge. This is a week long event where you get to play a random selection of levels while you are waiting for new content to be released. All players in Gaia's Challenge are put in a group of 20 players for the duration of the event.
Each level passed will give you one feather. Players are ranked on the leaderboard based on the number of feathers collected. At the end of the event the top players with the most feathers will earn a reward chest. The whole group is also working together towards a common Gaia's Gift reward. As long as your group hits at least one reward threshold, all the players that participated will receive a reward.