Today you can see in all over the world their are almost all devices are connected to one and another. Internet is also a computer network in which billions of computers and devices are connecting together.
Example
If their is a Router in your house and your computer, television and others devices are connecting with the router to share information.
So this is an example of internal network. And if your house devices are connected to the router and your router is connecting to the ISP and your ISP is connecting to the other large network then a big network is produced. So this network is called external network/internet. ISP means Internet Service Provider which can provide you internet.
Now we can discuss about why the networking skills is important for hackers:
If you become a professional hacker you must be know about the network and how a network work. You know about the network models, IP address and network protocols etc.
Networking skills is very important in the field of hacking. Almost all devices are connected to the network. Supposed if a hacker access to victim network. So he/she can do anything thing with the victim computers. Ethical hacker can also know about the knowledge of networking. So he/she can solved the problems if any bad hacker can attack on the network