With mood tracking enabled, team members are asked to (optionally) pick an emoji to describe how they are feeling at check-in time. The question looks like this:
The emoji will appear alongside their avatar in the dashboard and in the reports.
Answering the "How are you feeling?" question is not required by the team, but you can disable it altogether if you'd like in the Team Settings.
Note: the mood tracking feature is available in the free trial and in the Pro Plan and plans above it.