The HTTP X-Forwarded-Host header is the part of HTTP header that contains information for identifying the original host requested by the client. It is de-facto standard header.
Real life usage
Suppose you have multiple location dependent servers to serve your users, and a load balancer on front of it. In this case, host of load balancer might differ from the host address of servers. Also, host of servers may vary as per their location. Here, HTTP X-Forwarded-Host header can be used to identify which server does the user wants to use for his request. Normally, this is helpful in location dependent servers.
Here, host will be the host address of server.
To check the X-Forwarded-Host of any page you visited, you can go to Inspect Element then Network and then check the request header for X-Forwarded-Host.