This may get a bit technical.
The internet uses IP (internet protocol) addresses, there are version 4 and version 6. Virgin don't support v6, so we can ignore this.
1) Your router (the Hub) makes a local network, and gives each of your computers an
internal IP address.
2) The Hub itself has its own
internal IP address (typically 192.168.0.1)
3) The Hub is provided with a
public IP address by Virgin Media.
When you use an application on your computer, and connect to the internet, your computer connects to the Hub, and the Hub then retransmits the request to the internet service. The reply from the internet service comes back to the Hub, and the Hub then relays that to you.
This saves the number of Public IP addresses that your internet provider needs to supply, and given the world has run out of new version 4 addresses, this is a way of sharing 1 address for your home.
The Pubilc address changes, the internal IP addresses are given to your computers, laptops, phones by the Hub. The Hub can be set to always give the same internal IP addresses to a device.
Port Forwarding - this is how you tell the Hub what to do when something from the outside arrives that you haven't requested. Normally the Hub ignores these, but typically for gaming, you need to get these requests to the gaming computer. That is what port forwarding is about.
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