An excess is the amount that you have to pay towards the repair of your vehicle in the event of a claim for accidental or malicious damage, fire or theft. Alternatively if the vehicle is a total loss, this amount may be deducted from the settlement amount.
If the accident is deemed to not be your fault, you may be able to claim your excess back from the third party at fault as part of your uninsured losses, providing you have legal protection on your policy. You will be able to find out if this is part of your policy in your documents.