An endorsement is a written evidence of an agreed change to your policy. It is a document that incorporates changes in the terms of the policy. Any changes or alterations that you need to make can be done by way of an endorsement.
An endorsement may be issued at the time of issuing the policy to provide additional benefits and cover (e.g. legal liability to driver) or to impose restrictions (e.g. accidental damage deductible).
The wordings of those endorsements are provided by the insurance company. An endorsement may also be issued subsequently to record changes such as change of address, change of name, change of vehicle etc.