Google Cloud's identity and access management system. It lets you assign roles (primitive: Owner/Editor/Viewer, predefined, or custom) to members (users, groups, service accounts) over GCP resources.
In CDL it's fundamental to security. Principle of least privilege. Difference from AWS IAM: in GCP, permissions are assigned at the level of a specific resource or a project/folder/organization. Service accounts are identities for applications.
GCP IAM is a concept that can show up on the exam. Check the full guide to see it in context.