is a hardware/software standard that allows devices on a network to
communicate with each other and discover the capabilities available for
use. As a FrostWire user, what it can mean to you is that
FrostWire requires almost no manual configuration.
This is only appropriate if you are behind a router. If have only one computer, and it is wired to a vendor supplied internet connection, you probably are not concerned with UPnP. If you are using wireless in your home, or have multiple computers connecting to the internet, there is a good chance that you need to be concerned with UPnP OR port forwarding.
Most modern routers come with UPnP enabled, so it's likely you won't have to be concerned. If you have an older router, that does have UPnP capability, you may have to turn it on. For that, you'll need to refer to your documentation because NO two are alike. Once you figure out how to log into your router, (IP, ID and Password), you simply poke around until you find the UPnP setting option. Try advanced settings, or some such, first.
If you find that your modem does NOT support UPnP, then you will need to port forward. It's not that bad. It is a multi part effort but none of it is difficult. Find the port forwarding section in your router configuration. Again, you'll probably looking under advanced settings. Find the port forwarding section and bring up that page. Once I found mine, stepping through it was quite simple. You can't hurt anything, because you can delete any entry, so if you make a mistake, no real problem. A port above 1024 is a functional choice. Add and save your port forward specification. Exit the router. If you have multiple computers on your network, you'll need a port and forward for each of them. When you configure FrostWire, make sure that the ports and the computers match or it will not work. If you don't intend to run FrostWire on a given computer, there is noneed for a forward to that one.
Now, start FrostWire and go to tools>options>advanced>firewall and set it to manual port forward. Enter the port you chose to forward and click apply, OK and restart FrostWire.