Ordinarily, your physical damage coverage provides you with actual cash value settlement (cost of the vehicle minus depreciation) in the event of a total loss. By having loan/lease gap coverage, you avoid the out-of-pocket expense that would occur due to a "gap" between the vehicle's actual cash value and the lease or loan amount.
Coverage applies only to the original lease or loan agreement, and must be requested within a specific frame of time from the date the agreement is signed. Check with your independent agent for details. The coverage does not apply to overdue payments, previously un-repaired damage, or the transfer of lease or loan balances to other vehicles.