Reservations aren't Removed Automatically
All the expired reservations are automatically considered as REMOVED, but they will have PENDING status until someone will try to search for a reservation.
The Keep Tables Locked setting is used to calculate if a table is available for reservation or not. In case you are using the program with a Deposit to be left, the saved reservation will have a certain Status (given by the configuration of the payment method chosen). When the Status is Pending, it means that the reservation needs to be paid (or accepted manually from the administrator section) in order to become Confirmed. This setting defines the number of minutes for which the system should keep the tables of a Pending Reservation, as "locked" so that nobody can choose a table that is being reserved by another client.