UPNP is a technology to make router configuration easier. It makes it so devices inside your network can automaticlly configure the router to allow various inbound services (easpecialy for online gaming).
There is a problem in many routers that exposes the UPNP service to the Internet, making it possible for someone on the Internet to configure your router to allow themselves access into your network. Of course, this would be a very bad thing to have happen.
You can check easily for this flaw by doing this:
Most likely, you'll get a good result and you don't have to do anything more!
But if you get a bad result, you need to do something as soon as possible. The resolution will be to do one of these (in order of least expensive to most expensive, provided you can do it yourself):
And then you should test it again to make sure the problem is gone (if it's not, you should try one of the other options). If you don't know how to do these things and are unable to solve the problem, you'll want to hire a tech to do it for you. There is a lot more information about this problem at grc.com so feel free to read about it there or ask me about it.
If you're one of my customers and I setup your router, I've probably disabled UPNP anyway because I never trusted it and have been turning it off for several years.
Unfortunatly I've heard that in some routers, even turning UPNP off leaves it on and exposed to the Internet, so it's still worth testing.
Good luck!