In each alert level, you can program instructions for your military and/or civilian dwarves to follow. The game contains two alert levels by default — Inactive and Active/Train. In Inactive, all squads are assigned no orders. In Active/Train, all squads are supposed to be assigned to train the entire year.
Alerts are created and managed on the Military screen.
A civilian alert will automatically confine all non-military dwarves to the burrows defined in the alert for as long as the alert is active. When you set the civilian alert level to an alert that includes a defined burrow, your civilian dwarves will ignore their normal burrow-based job restrictions even if their burrow is one of those in the civilian alert.