An endorsement is a written proof of an agreed change in the policy, as and when the need arises. There can be two types of endorsements: premium bearing and non-premium bearing.
In a premium-bearing endorsement, you will be charged an additional premium for things like transfer of ownership, addition of LPG / CNG kit, change of RTO location etc.
In a non-premium bearing endorsement, you won't be charged anything for the changes incorporated in the policy. For example, correction in contact details, correction in name, correction in engine / chassis number, addition of hypothecation, etc.