A Compute Engine feature that ties VMs’ Linux accounts to Google identities and controls SSH access through IAM roles (Compute OS Login and Compute OS Admin Login) instead of SSH keys in metadata.
On the ACE it is chosen when you want to revoke SSH access by removing a role and audit who connects. It is enabled with the enable-oslogin=TRUE metadata or an organization policy.
OS Login is a concept that can show up on the exam. Check the full guide to see it in context.