Check Open port allows you to troubleshoot your port forwarding issues. This online port scanner gives you instant and verified results. You can check if the port is open from the internet using this online utility. If you are running a webserver, you need to ensure port 80(HTTP) is open on your IP which can be verified from CheckOpenPort.com. In addition to this if your router does not support UPnP you will need Port forwarding to make some of your applications to work.
Port forwarding allows remote computers to connect to a specific computer or service within a private local-area network (LAN)
When you want to expose a service available on your Computer to the internet. Let's say you setting up a Minecraft Server to play with your friends.
You will need to modify your router configuration to allow Port Forwarding. The easiest way would be to search Router Model Number and Port Forwarding on google.Further Reading on Wikipedia