Just to make it as beginner-friendly as I can:
In your house, you are on a local LAN, and all your computers have very similar local IP addresses, often 192.168.0.xx, with just xx being different for each PC, and between 0 and 255. These are local addresses only, good for talking to a local PC from another local PC, but they can't be used on the wider Internet.
To get on the Internet, you need a different address, with the same xx.xx.xx.xx format, but different numbers. That address is supplied by your Internet Service Provider, and grabbed by your Internet box, which acts has a bridge and router between your local network and the Internet.
When one of your PC gets on the Internet, your internet box sees the request, passes it on to the Internet, gets the result, and passes it back to the original PC.
When you want to get to your Pi or another home PC from the Internet, the only Internet address you have is that of your box, and upon getting your "foreign" request, your box has no way to know to which PC to forward it to. Hence, you need to set up "port forwarding" in your box/router, so that it knows that Internet stuff coming to port yyyy should be passed on to which PC.
To achieve that you need 2 things:
1- to know your "public" IP adress. That's the one reported for your Pi and Mint PCs by sites such as whatismyIP.com. It will be the same for both PCs.
2- to know which port you need to forward to your local PC. Your box will have an admin interface that will let you first, accept "packets" (info) that comes in a specific port or range of ports, and second, define to which PC and which port to pass it on to. So you need to know the local IP adress of you Pi, and the port which VNC uses (someone listed possibilities earlier). In your Internet box's admin interface, you forward that port to the same port on your PI's local IP adress. Then you save and reboot your internet box for the settings to take.
- Most ISPs will always give you he same public IP address, but some ISPs will change it from time to time, or even daily. Let's assume you have a fixed one (otherwise you'll need to use a dynamic DNS service)
- your local IP address may also change from time to time , depending on what your Internet box does. Again, usually they don't, but if they do there are ways to force your Pi to always use the same IP local address.
- for safety reasons, you may want to NOT use VNC's default port. That means changing the forwarding rule in your router, so that it listens to , say, port 12345 instead of port 5900, but then stills forwards it to port 5900 on your Pi; then to connect to your Pi from a foreign PC, you specify the 12345 port on top of your public IP adress, either in the "port" field if your VNC client has one, or by adding ":12345" after your public IP adress, ie 126.96.36.199:12345 in your clients "IP adress" field.
Hope this is understandable ^^