The act of opening a port, and choosing a device on the network to forward those requests to, is called port forwarding. You can think of port forwarding like attaching a pipe from the router to the device that needs to use the port — there's a direct line-of-sight between the two that allows data flow.
For example, FTP servers listen for incoming connections on port 21. If you have an FTP server set up that nobody outside your network can connect to, you'd want to open port 21 on the router and forward it to the computer you're using as the server. When you do this, that new, dedicated pipe is used to move files from the server, through the router, and out of the network to the FTP client that's communicating with it