Upnp allows the program to automatically connect to the router and set the ports up without you having to do it manually. Only works with routers that support upnp and pretty much routers these days do, unless you have a very old router. Essentially though, upnp helps setup all the ports with the router to allow it to connect. Here is a link that may better help you understand what upnp is
uPNP does create a lot of issues. It wrecked my Skype under Windows 8 entirely and I believe people should disable it, and Skype installation should as a default have it disabled.