It's a statement in a periodic - most commonly per annum - report of the status of an organization that claims a government has not requested, received, or intercepted certain information about the organization. If the line is altered or removed then the government has accessed the organization's information.

If the admins are coerced by a law enforcement agency to reveal user info with a gag order, it won't be posted. AFAIK, law enforcement can get orders to compel someone to not say something, but not force them to keep posting the canary.

There's other stuff in there too, but it's basically the canary in the coal mine that dies around the time you should be GTFO'ing.

Oh here ya go..

https://googlethatforyou.com?q=what%20is%20a%20warrant%20canary