If you are considering renting a car then you should be aware of the costs involved if you damage it. All rental companies should go through the rental agreement with you in detail prior to you signing it and if there is anything you do not understand, that is the time to ask.
Most rental cars carry an excess that you will have to pay should you cause damage to the vehicle. This includes damaged caused to the vehicle by a third party if you cannot claim through their insurance i.e they do not admit liability or you didn’t see who damaged it.
Some rental companies will offer you an excess waiver which is usually an amount added on to the daily rental of the car, meaning should you damage it, you do not have to pay the excess. Again, read the agreement thoroughly and if you cannot see anything about excess and damage costs then ask prior to signing anything.