Heartbeat enables the highly available operation of services. In case of an error, a backup system takes over the task performed by the malfunctioning computer and the service remains available. This is made possible with the help of a heartbeat signal, which the computer systems exchange via a network connection. If the notification is not sent by a computer, the functioning computer assumes that its counterpart is no longer available (e.g. due to a defect) and subsequently takes over its task. The program is part of the Linux-HA (High-Availability Linux) project. Heartbeat is free software and runs on all popular unixoid operating systems.
You can find more information on the project page.