Amounts set on a billing account (or some projects or services) with thresholds that send alerts on actual or forecasted spend. They do not stop resources: to act automatically, notifications are sent to Pub/Sub.
A frequent ACE trap: a budget does not cap spending. To cut it off you must automate, for example with a function that disables billing when it receives the Pub/Sub notification.
Cloud Billing budgets and alerts is a concept that can show up on the exam. Check the full guide to see it in context.