Health check function checks whether the load balancer and the target server are connected. It check every certain time whether we can see the website via load balancer.
For AWS, it is judged whether it can access the health check URL set on the console screen. It is OK if the HTTP status 200 is returned from the URL. If inadvertent 404 comes back, even if you access the URL set by the load balancer, the the site will not displayed.
It seems that it becomes OutOfService somewhat when restarting the EC2 instance in the state attached to the ELB. Also, as it is said that 200 should be returned by health check, if you do not do BASIC certification naturally, it will be OutOfService. 401 error comes back. If you are using ELB and need to make Basic authentication, only URLs for health checks must be in a state where authentication is ineffective.
