Forwarder is a Spigot/PaperMC plugin designed to get around the limitations imposed by hosting services on internet ports.
Some plugins, such as DynMap, create a local http server on which you can view the world map; however, if the hosting service only accepts connections through the port used to connect to the game server, DynMap will be unusable.
The goal of Forwarder is to solve this problem: using the simple command /fw add, you can create a tunnel (both http and tcp) through which you can connect remotely.
Note: currently, if you make any changes to the forwarder configuration, you must restart the server to apply them.