An Additional Insured is a party added to a liability policy to extend coverage with regard to a specific job, activity, or location. The person or entity has a business relationship with the named insured that creates a risk of lawsuits against the named insured.
This is often required by a client, property owner, or bank as a means to protect their interest in the event that a covered claim or suit is brought against them for injury or damage caused by your negligence.