Question
What is a certificate of insurance?
Short answer
A one-page summary that proves your business has insurance: which policies, with which insurer, the limits and the dates. Clients and landlords often ask for one before you start work.
What it shows
A certificate of insurance lists your insurer, policy numbers, policy dates and limits for coverages like general liability, car, workers' compensation and professional liability. In the US it's usually on a standard form called ACORD 25.
You get it from your insurance agent, broker or insurer's website, usually free and within a day.
A certificate isn't coverage by itself
It only describes your policy. If a client wants to be covered by your policy for claims from your work, the policy needs an additional insured endorsement naming them. Writing their name on the certificate alone doesn't do that.
Before you send one, check that the limits match what the contract asks for and that the dates cover the whole job.
This is general information about how US policies usually work. Your own policy's wording and your state's rules decide the details.