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 Rever user can participate in them.
Challenges recognize those users who generate the most points with their ideas. This means that you do not necessarily need to have the best idea. The more ideas you enter in the challenge, the more points you will get.
How do I participate in a challenge? to start capturing ideas within one.