An Endorsement is a form that is attached to a policy in order to modify the policy terms. An insurance policy is a contract and in it's basic form includes terms of service, policy conditions and other agreements between the insurance company and the first named insured. Un-modified policies are rare.
You'll find the standard workers compensation policy is made up of:
The Information Page
The General Section
Part One - Workers Compensation
Part Two - Employers Liability
Part Three - Other States Insurance
Part Four - Your Duties If Injury Occurs
Part Five - Premium
Part Six - Conditions
The basic policy contract must be modified for many reasons. One of those reasons is to comply with individual state requirements. This is accomplished by the application of endorsements to the policy. Here's a few of the typical endorsements used to bring a workers compensation policy into an individual states compliance, just a few samples:
Arkansas - Arkansas Amendatory Endorsement WC 03 06 01 A - This Arkansas Endorsement modifies under Part 2 Employers Liability section C2 Exclusions. It also modifies Part 6 Conditions D Cancellation - by replacing the standard policy wording with specially approved Arkansas wording.
Kentucky - Kentucky Cancellation and NonRenewal Endorsement WC 16 06 01 - This Kentucky Specific Endorsement modifies the standard cancellation and nonrenewal sections of the policy conditions and replaces it with special conditions that apply only in the State of Kentucky.
There are many other reasons that a policy may need to be modified. Some may require the policy conditions to be modified because of a specific job contract. A typical modification for contracting risks is that of waiver of subrogation. While not allowed in all states there is a standard endorsement available for use in those that do allow waivers to apply. For example:
Illinois - Waiver of Our Right To Recover From Others Endorsement - WC 00 03 13 - This is a non state specific endorsement that is commonly used when an employer needs a Waiver of Subrogation on their workers compensation policy.
So an insurance endorsement is a special form that is added to a workers compensation policy that in some way changes the policy. They sometimes add coverage or conditions and at other times reduce or remove coverage or conditions.
It's very important that you always read your insurance policy! Don't forget, an insurance policy along with it's endorsements and individual policy parts is a contract!