An excess is the amount you have to pay if you make a claim on your insurance. The excess you pay will be detailed on your policy documents.
When you get a quote you can choose to increase the excess, which will help to reduce your monthly payments. You are responsible for paying the excess as detailed within your policy document. You also have to pay the first £250 of a claim for damage to property under the Third Party Liability section in respect of dogs only (if you have this cover).