When you’re playing an online game like Fortnite, your computer is constantly opening and closing connections to the server. In most cases, it does so using a system known as UPnP (Universal Plug and Play). However, no automated system is perfect, and in some very rare cases, UPnP can get tripped up by your network settings or the game’s server configuration, causing higher latency than you would normally see.
In cases like these, port forwarding could help get you back on track. Essentially, port forwarding routes your traffic through ports that are permanently open instead of opening and closing them as needed. Most people will see no difference from port forwarding, but if you’re having problems playing Fortnite at all because of lag, it doesn’t hurt to try.