Alerts are in the Alerts menu. They notify you when a metric crosses a threshold, without changing anything in your campaigns. Click New Alert and fill in: the Metric (CPA, CPC, CPM, CTR, Daily Spend or ROAS), the Condition (Greater than, Less than or Equal to), the threshold Value, the Ad Account (leave empty to monitor all of them) and, optionally, the Campaign objectives. Then click Save Alert. When the condition is met, a notification arrives in the dashboard bell with the metric's current value and the account. Alerts notify, Automated Rules act: if you want to pause campaigns or change budgets automatically, use Rules.

When you create the alert, also choose the level: account (default), campaign, ad set or ad. At levels below account, the condition is tested item by item, and the notification tells you exactly which item fired.