Port forwarding is allowing a port from a device/computer on your local area network (LAN) to cross over to your wide area network (WAN). WAN is internet, anyone online will pretty much be able to connect to your WAN ip address and use the port you have forwarded through your router, just as long as your internet service provider allows that port to be forwarded and of course if the person who is connecting to you has the necessary client to receive/send data from/to the server that you're forwarding.
What is a port?
i don't know much about ports, but what i can tell you about them to give you a basic understanding about them is that their a number associated with your IP address either lan/wan. 123.456.789.101:6666
"6666" would be the port in this address. most of the time the port is a part of the address, unless you include a protocol such as "HTTP://" before the ip address. if no port is defined for any address if before the ip is HTTP:// then the assumed port number is "80"
hope that gets you started, because thats not even close to what there is to know about that :P