In the category 'Challenges', you will find challenges of drills that are present in the group(s) that you are part of. These challenges can vary depending on the parameters that are set by the administrator of that group.
For example, the administrator could have determined that it is necessary that you reached a certain competence level before a set date. It is also possible that a permanent challenge has been set on a set of drills in the group. This means that you are required to maintain a certain level of knowledge of the drills while you are a member of that group.
Challenges are useful because they help you get to the correct level of knowledge of a subject in a limited period of time. These challenges are set by the group administrator. The administrator can also link notifications to the challenges. For example, the administrator can set the communication so that you receive a notification and/or email when you have a new challenge, but also that you receive a notification when you are not on schedule.