Definition - What does Port Forwarding mean?
Port forwarding is a networking technique through which a gateway or similar device transmits all incoming communication/traffic of a specific port to the same port on any internal network node. Port forwarding enables an external source network or system to connect to an internal source node/port, which typically connects to Internet services and an internal private LAN.
Port forwarding is also known as port mapping, tunneling or punch through.
Techopedia explains Port Forwarding
For example, suppose that a router receives a packet with an IP address and port number in the packet header. If the router is not configured with port forwarding, it will first resolve/identify the port before transmitting. However, if port forwarding is configured, it will automatically transfer the packet internally to the destination node. The whole process of port forwarding is transparent to all connected network clients.