The discuss screen is used for internal discussion about the time details from the approver to the TimeTracker user or vice versa.
If the TimeTracker user has not put the correct time details, the approver can comment in the discuss screen.
The TimeTracker user can respond to the approver from the comments screen, make the necessary changes to the time entry and submit it for approval.
The Approver and the TimeTracker user can see the comments when logged in from their Salesforce app or from the Web app.