A service that enforces constraints on how resources can be configured in the organization, a folder, or a project; for example, gcp.resourceLocations to limit regions or compute.vmExternalIpAccess to block external IPs. It supports a dry-run mode.
On the ACE it is contrasted with IAM: IAM defines who can do something, while organization policy defines which configurations are allowed for everyone. It is inherited and can be customized at lower levels to create exceptions.
Organization Policy is a concept that can show up on the exam. Check the full guide to see it in context.