Why are items on my staff schedule highlighted red?

An item highlighted in red on your staff schedule notifies you when a staff member has requested a cancellation of a shift. By hovering your mouse over that entry you’ll see additional information about the cancellation, including the date cancelled and any reason provided by your employee.

To accept the cancellation you can right click on the entry and click "Accept". In the same way, to decline the request, click "Decline".