This is the amount you will need to pay in the event of a claim. For example, if you have a £100 excess then you will pay the first £100 of the claim you make.
Your excess can be made up of both a compulsory excess (set by the insurer) and a voluntary excess (chosen by yourself). For example: you may have a £100 compulsory excess and a £150 voluntary excess giving you a total excess to pay in the event of a claim of £250.
You can see the excesses which apply to you in your policy documents.