An Additional Insured is usually your landlord (someone you hold a lease with), an employer, board members, or an agency you contract with. Adding them to your policy gives that entity protection (an identical set of limits of liability) under your policy. These limits will be used in the event that they are sued for a Professional Incident that arises from services that YOU provide while under contract with them.
NOTE: A business partner, employee, sublessor, intern you are supervising, or a corporation that you own CANNOT be added as Additional Insureds. These cases may require that each individual take out their own coverage or that a group/corporate policy be started. In no way will the policy cover Professional Incidents that arise out of acts, errors, or omissions made by the Additional Insured entity.