UPnP or NAT-PNP support
UPnP and NAT-PNP protocols provide automatic port configuration features for various routers (gateways). Some routers have these features enabled by default, some have not. It allows apps on your device to open ports on your router when needed and to close them when they are stopped.
UPnP might be convenient, but it adds its potential security issues. It assumes that every device on your local network is trustworthy. So if you happen to get infected by malware that wants to initiate a direct connection with a remote attack, your UPnP router will allow it without question. Such a connection would be much more difficult to open with UPnP disabled.