Failover means switching to another machine when one of the machines fails.
Failover is an important technique in achieving high availability. Typically a load balancer is configured to fail over to another machine when the main machine fails.
To achieve the least downtime, most load balancers support a feature of heartbeat check. This ensures that the target machine is responding. As soon as a heartbeat signal fails, load balancer stops sending the request to that machine and redirects to other machines or cluster.