If we go to SETTINGS > BOOKING SETTINGS > RESTRICTIONS, we can see various Booking Customizations.
Under the 'Restrictions' tab, if we scroll towards the bottom, we can see:
Outstanding Reservations is the total number of outstanding reservations you can have at one time (i.e. the number of reservations you have 'pending' that have not happened yet).
Concurrent Reservations are the total number of reservations you can make during the same timeframe (ex. having a reservation from 3-4pm and the 4-5pm or 3-4pm and then 3:45-5pm).