UPnP comes enabled by default on many new routers. At one point, the FBI and other security experts recommended disabling UPnP for security reasons. ... UPnP stands for “Universal Plug and Play.” Using UPnP, an application can automatically forward a port on your router, saving you the hassle of forwarding ports manually.

What will happen if I disable UPnP?

If you turn off UPnP altogether, your router will ignore all incoming requests so you'll have to set up devices manually. This means that the router will no longer automatically open ports on your LAN, ignoring even legitimate requests.

Should UPnP be on or off for gaming?

Many games work fine by simply relying on UPnP (Universal Plug and Play) to open a port when it's requested by your machine and then close it when the application is done. Before you worry about opening ports, find the UPnP option in your router and make sure it's enabled.