domain.com is a generic reference to my own domain http://www.jason.com for example that I have to access on my local network applications.
I am using Caddy Server as software that serves as my webserver. So instead of having to type in the external ip address I can type in my domain name for example http://www.jason.com/opensprinklerhome and that will translate that to my external IP address:port number internally to reach my home opensprinkler on 192.168.1.119:6119.
I hope that makes more sense I not sure if I am explaining this technically correct.