What is Heartbeat?

A heartbeat is a small "I am alive" message that a server sends out on a fixed schedule. As long as these messages keep arriving, the rest of the system knows the server is up and working. When they stop, the system starts to suspect that the server has failed.

Think of a daily check-in call. An elderly relative calls you every morning at nine to say all is well. If one call is missed, you do not panic; maybe they overslept. If the silence continues for two or three mornings, you send someone to knock on the door