A set of Google Cloud recommenders that analyze actual usage to suggest improvements: stopping or deleting idle VMs, rightsizing oversized VMs, or trimming IAM roles with unused permissions.
On the ACE it is used to optimize costs and permissions. The IAM role recommender analyzes the permissions used in the last 90 days.
Active Assist is a concept that can show up on the exam. Check the full guide to see it in context.