An additional insured is a person or organization, that you are doing business with that is added to your insurance policy.
By adding an additional insured, you are extending coverage on your policy to protect the person or organization from claims resulting from your negligence.
For example: A suit from a customer eating spoiled food. Since the lawsuit would sue you, and most likely the venue, your policy would help protect both of you.
The additional insured will want to be shown proof of being added. This is accomplished a standard form called an Acord 25. Some agencies will charge extra for this work; we do not. We also offer a form for the information needed to generate the certificate for you here.