This is a different address then your network (or internal) IP.
The external IP is the address where you go online with. If you have several computers IN the network which have all their own IP's, they all connect to the internet with only one IP:

There are different reasons why it could be useful to know what your external IP is. For example if you want to setup a remote desktop connection, you need to type the external address to connect to your home network.



Another reason can for example be if you are planning to setup a FTP server.
But there are several more reasons why it could be useful to know your IP and that's why the link below is a very useful link.

You can get your external IP by clicking on the link below.