The challenges are pooled in the Challenger tab. They are contests where your ideas have a specific goal. They are launched by the organization you belong to, and any one can participate in them.
When creating a Rev, you can link it to the current Challenges that the organization has launched, capture all the Revs that you think are necessary within the Challenges that are aligned to your idea.
If you want to learn more how to participate in a Challenge click here:
Participate in the Rever challenges
If you are a Coach or Champion and want to learn how to launch a Challenge: