What it is
A canary is a short signed statement published on a schedule, often including a recent date or news headline to prove it is fresh.
Reading it
Check the signature, then the date. A valid, recent canary means the operators could still sign messages on that date.
Overdue canaries
A missed update is not proof of trouble, but it is a reason to be extra careful with any new address.
Limits
A canary says nothing about whether a particular link you were sent is real. You still have to check that separately.
A fresh signed canary is reassuring; an overdue one is a reason for extra care, not panic.