An excess is an amount that you pay as a lump sum in the event of a claim.
Only after this amount has been reached will your insurance pay out.
Imagine you have a damage on your own vehicle.
Assuming the repair costs are 800 €. If you should have chosen in this exemplary case to have an excess of e.g. 150 €, your insurance pays the remaining 650 € of the costs of the damage.
An excess usually creates a lower insurance premium. However, it always depends on your personal insurance package.