At the time of collection of the vehicle, the rental company may retain a deposit, which is called a security deposit, to cover possible losses, as well as damage to the vehicle, fines or speeding.
The amount stipulated as a deposit will be temporarily blocked/charged to the main driver's credit card and held for the duration of the rental period.
Provided the vehicle is returned undamaged at the end of the rental, the deposit will be released / refunded.
On the other hand, the excess is the maximum amount you would have to pay in case of damage to the vehicle.
You can find all the information about the amount of the excess for each offer in the terms and conditions and on your rental voucher.