The challenges are things you have to do in order to get a reward. By completing them can earn extra chips and XP (experience points).

You can check what challenges you have, what you need to do to complete them and what you gain from them from the Cup button in the upper right corner of the platform.

When you complete a challenge, a "Get Reward" button will appear to the right of it.

 

As soon as you do, the level of the challenge as well as its condition will change.