A set of identical VMs created from an instance template. It provides autoscaling, autohealing with health checks, and rolling updates, and it can be zonal or regional (spread across zones).
On the ACE: instance templates cannot be edited, so changing the machine type means a new template plus a rolling update. A regional MIG protects against a zone failure.
Managed instance group (MIG) is a concept that can show up on the exam. Check the full guide to see it in context.