What is the Uplink port on a router, hub, or switch for ?

An uplink port on a hub/switch/router (or any other network device) is simply an Ethernet port where the Receive (Rx) and the Transmit (Tx) signals are swapped by crossing the Transmit and Receive twisted-pair wires.

This lets the Transmit signal (and twisted-pair) on one device correctly connect to the Receive connection on the other device and vice-versa, allowing for stacking similar devices, or simply expanding the number of ports.

The Uplink feature is often implemented by a shared port, meaning one port with two different jacks (usually next to each other). Note that you should never plug something into both the normal and shared uplink port at the same time in such devices ! Some newer devices implement "auto MDI/MDI-X", a feature that automatically figures out how to connect with whatever is plugged into it.

An uplink port is essentially a substitute for a crossover cable, it has the same functionality.