A canary is a safeguard against dangers. In my example, my daily emails from my investment account is my canary. When I stop getting emails, I know there's a problem.

I cannot remember the details, but one of my favorite canarys was a website that had a paragraph that basically stated we have not been compromised in the past 24 hours. It had a timer that had to be rest daily or the paragraph whould disappear from the website.