CPU and memory, allocated per request or always-on.
Cloud Run charges for the CPU and memory allocated to your container while it is handling requests — or, in instance-based billing, for the whole time an instance exists. Which mode you are in is the single biggest factor in a Cloud Run bill, and minimum instances put you effectively in the second one.
Billed: Per vCPU-second and gibibyte-second, plus a per-request charge. Billing differs sharply between request-based and instance-based modes.