Google Cloud’s unified agent installed on VMs to collect logs (system logs and apps such as Apache or MySQL) and metrics such as memory, sending them to Cloud Logging and Cloud Monitoring.
On the ACE: without an agent, Compute Engine only shows hypervisor metrics (CPU, disk, and network). If memory or application logs are missing, you install the Ops Agent.
Ops Agent is a concept that can show up on the exam. Check the full guide to see it in context.