In a production application you usually have many servers, and each of those servers gets checked periodically to make sure they're still healthy and working as expected. When they are, requests can be routed at them by your load balancer. If a server doesn't respond to the healthcheck, then it is presumed to be dead or unhealthy, and requests are diverted to the healthy servers instead. If you've got an autoscaling solution set up, unhealthy servers can be killed, rebooted, and re-added to the load balancer's pool of healthy servers.
