Short for Universal Plug and Play, UPnP is an architecture that simplifies networking by allowing wired and wireless devices to communicate without proper configuration and minimal setup. For example, plugging a UPnP device into a computer with an operating system with UPnP support means the device is automatically detected and works.
If you are having issues with UPnP or connecting devices to your home network, try updating the firmware on your router. See: How to update a router firmware.
All Windows users can use the UnPlug n' Pray utility to quickly enable and disable UPnP.