Heartbeat is a Linux subsystem for high-availability under Linux.
For example : Heartbeat can be used with DRBD and Pacemaker.
With Heartbeat, your servers will send a packet to your other servers periodically to detect "in real-time" servers that would become inaccessible.
If a server sends no more packets, it means that this server is down or that its network card is failing.
Note that you will need 2 communication paths (or 2 network cards) to implement this technology in the best conditions.
This also applies to other high availability systems.