By default, Azure Application Gateway probes backend servers to check their health status and to check whether they're ready to serve requests. Users can also create custom probes to mention the host name, the path to be probed, and the status codes to be accepted as Healthy. In each case, if the backend server doesn't respond successfully, Application Gateway marks the server as Unhealthy and stops forwarding requests to the server. After the server starts responding successfully, Application Gateway resumes forwarding the requests.
Note: The default probe request is sent in the format of <protocol>://127.0.0.1:<port>/. For example,
http://127.0.0.1:80 for an http probe on port 80. Only HTTP status codes of 200 through 399 are considered healthy. The protocol and destination port are inherited from the HTTP settings. If you want Application Gateway to probe on a different protocol, host name, or path and to recognize a different status code as Healthy, configure a custom probe and associate it with the HTTP settings.
Reference:
https://docs.microsoft.com/en-us/azure/application-gateway/application-gateway-backend-health-troubleshooting
