The excess is the amount you pay towards each claim before your insurer covers the rest of the eligible vet bill.
If your policy has a £100 excess and your vet bill comes to £600, you pay £100 and your insurer pays £500, assuming the treatment is covered and you haven't hit your policy limit.
Excesses come in different forms. A fixed excess is a set pound amount per claim. A percentage excess means you pay a proportion of the total bill rather than a fixed figure, so on a large claim a percentage excess can add up to considerably more than a fixed one. Some policies have both: a fixed amount plus a percentage on top.
A higher excess usually means a lower premium, because you're taking on more of the risk yourself. A lower excess means you pay less when you claim but more each month. Neither is automatically better. It depends on your situation and how often you're likely to need to claim.
One thing a policy detail to check is whether the excess applies per condition or per policy year. A per-condition excess means you pay it every time you claim for a new condition. A per-year excess resets annually regardless of how many conditions you claim for, which can work out cheaper if your pet has multiple issues in the same year.