A challenge is simply an activity you have to complete to progress to the next level, for example; win 3 times in a row on any slot game or finish top 100 in the daily tournament. If you complete a challenge you will be rewarded with a cool badge. All your badges will be stored in the “My trophies” section.
In the big image shown at the top of the start page you will see a number of treasure-chests. If you’re on mobile you will be able to swipe between the chests. You can tap/click any of the chests to open up the challenge description. The description will explain how to complete the challenge.