Emails you when a campaign that should be running flips into a non-serving state - Google calls this misconfigured or not eligible - and includes Google's own reason for it. It only fires on the change into that state, so it won't nag you about a campaign that's been off all along.
Trigger states - which campaign statuses count as stopped. By default, the two serious states Google reports: misconfigured and not eligible.
Limited - a softer "running, but held back" state; off by default because it's noisier.
Checks every 15 minutes; 7-day cooldown per campaign.