What does it mean that a shift is going to 'expire' on a certain date and time?

This occurs when a shift will no longer be visible to the candidates to apply via the app. The default is set so that a shift is visible up until an hour before the start of the shift for weekday shifts, or up until 5pm on the Friday for weekend shifts. This is in order to give the managers enough time to approve any applications received. When you are posting the job listing you can change this default option. 

If a candidate has agreed to complete a shift after it has expired, they can complete an ad-hoc timesheet.