Additional insured status extends an insurance policy's liability coverage to a party other than the named insured. On construction projects, owners and general contractors routinely require subcontractors to add them as additional insureds on the sub's commercial general liability and excess policies. The endorsement allows the upstream party to tender a defense to the sub's carrier when a claim arises from the sub's work.
Not all additional insured endorsements are equal. ISO form CG 20 10 covers ongoing operations only. CG 20 37 covers completed operations. CG 20 38 covers both, plus contractors. Owners and GCs typically require both ongoing and completed-operations endorsements with primary and non-contributory wording. Verify the actual endorsement form on the certificate of insurance and request a copy of the endorsement, not just the COI.