An additional insured is a person or organization added to your insurance policy who receives coverage under that policy for liability arising from your operations. When a general contractor requires you to name them as an additional insured on your CGL policy, they are asking for your insurance to protect them against claims that result from your roofing work. This is one of the most common contractual insurance requirements in construction.
Additional insured status is granted through an endorsement attached to your policy. The most common endorsements are CG 20 10 (for ongoing operations) and CG 20 37 (for completed operations). Together, these provide the GC with coverage both while you are working on the project and after you have completed it. Some policies use a blanket additional insured endorsement that automatically extends coverage to any party required by written contract, eliminating the need to issue individual endorsements for each project.
When a GC requires this on your COI, it means they want your insurance to respond first if a claim arises from your work, protecting their own loss history and deductible. The additional insured's coverage under your policy is limited to liability caused in whole or in part by your acts or omissions. They do not get coverage for their own independent negligence under your policy. Make sure your policy includes additional insured endorsements before signing contracts that require them, and confirm with your agent that the endorsement form matches what the contract specifies.