Each policy normally comes with a compulsory excess, plus a voluntary excess if you've selected one. A compulsory excess does what it says on the tin. If you make a claim, it's the amount you pay towards the cost. If your TV was stolen and you made a claim for £500, and had a compulsory excess of £50 – a typical sum – you'd get £450.
If you have a £50 voluntary excess and a £50 compulsory excess, you'd only get £400 for the same claim.
The provider will tell you how much the compulsory excess is when you buy the policy.
What you then choose as a voluntary excess on top (if any) is entirely up to you. A higher voluntary excess will lower the cost of your annual or monthly premium, as the insurer pays less if you have to make a claim.
Think carefully. If you can't afford to cover a large chunk of the cost if you need to claim, opt for a lower voluntary excess.