Most policies have a standard excess which means you agree to pay the first part of any claim, for example the first £100 or £200. If you agree to a higher excess you might get a cheaper insurance premium. This is called a voluntary excess.
The insurer may impose an additional excess over and above the standard excess if they feel the risk is too high. For instance, if you are a young driver (under age 25) your car insurer may well impose an additional excess.