In a challenge, you're tasked with collecting a number of specific types of Moments within a given time frame. Normally, you'll need to collect somewhere between 9 and 14 Moments, and you'll have between one to three weeks to complete the challenge. Once you have all the required Moments, your job is simple: hold on to those Moments until the challenge countdown timer expires.
There are usually at least a couple challenges running at any given time, so head over to the challenges page to check your progress and seek out your rewards. Don't forget that you can fill in any missing slots by shopping in the marketplace, or visit the Discord community to see if you can make any trades to help round out your collection.
For more information on challenges, check out our blog post here.