A Certificate of Insurance (COI) is a standardized snapshot, almost always issued on an ACORD 25 form, that a subcontractor's insurance agent prepares to prove coverage is in force. It lists the carrier, policy numbers, coverage types, limits, and effective and expiration dates.
For a general contractor, the COI is the front-line document for confirming a sub meets the insurance requirements in their contract before they set foot on site. Because a certificate is only a summary and confers no coverage by itself, careful GCs pair it with the underlying endorsements and re-check it at every renewal.