Limits on the amount of resources or requests a project can use, such as vCPUs per region or API calls per minute. They are viewed in the console or with Cloud Quotas, and you can request an increase.
On the ACE it comes up when a deployment fails for lack of quota: the fix is to review the quota and request an increase. Spot VMs consume quota too.
Google Cloud quotas is a concept that can show up on the exam. Check the full guide to see it in context.