If your business is lucky enough to be inundated with customers wanting to make bookings with you, it can happen that two people select the same time slot, enter their details and click confirm. Previously, the first person to click confirm would get the booking and the other would be encouraged to choose another time.
The "reservations" feature will mean that from the moment a customer selects a timeslot on the availability step of your customer-facing booking tool, this slot is "reserved" for them for 7 minutes (you can change this time in your settings).
If the customer doesn't complete the booking process and book in 7 minutes, the time slot will be "released" for anyone else to book.
Additionally, when a customer is making a booking, you'll see the booking coming in on your internal Appointedd calendar: