Endorsements are how insurers customise a standard policy for individual circumstances. They appear in your policy schedule and they modify the standard terms in a specific way.
The most common endorsements in pet insurance are exclusions added because of something in your pet's health history. If you declared a pre-existing condition when you applied, the insurer may add an endorsement to your policy excluding that condition from cover. So your policy might say on the schedule: "This policy excludes any condition relating to the hip joints" or similar wording.
Endorsements can also be added at renewal if your pet has been treated for something during the policy year. Its not automatic but its common enough that its worth checking your renewal schedule carefully each year to see if anything new has been added.
Less commonly, endorsements can add cover rather than remove it, for example if you've negotiated with your insurer to have a previously excluded condition reconsidered after a period with no symptoms.
"Check your renewal schedule carefully each year to see if any new endorsements have been added."
The important thing about endorsements is that they're specific to your policy rather than applying to everyone on the same policy type. Someone else with the same policy from the same insurer might have completely different endorsements, or none at all. Its your schedule that tells you what applies to you.