Port forwarding or port mapping is an application of network address translation (NAT) that redirects a communication request from one address and port number combination to another while the packets are traversing a network gateway, such as a router or firewall.
Services running on a computer behind a router are not to be reached from the internet, unless port forwarding rules are defined on the router. Many software programs will attempt to open ports in the router automatically via UPnP. If UPnP is disabled, rules can be added manually and some routers offer the option to specify the source IP address or netblock that can connect on a specific port to reach a certain service inside the network.
Combine port forwarding with IP filtering. For example, if you want to access a server on your home network from elsewhere, create a port forwarding rule for a port in the router, but only allow connections from the IP address or IP netblock of the elsewhere.