What he's doing is setting up a secure tunnel between his remote computer and home computer/server, via SSH, and forwarding ports. This let's one use SSH to access all sorts of services, on the server, without having to open up those services' ports in your home router/firewall/etc, thus making things more secure. People do this for all sorts of things: accessing a home FTP server, a website, a home email server... all without opening up those services to the outside world.
I've never done it to specifically use Apple's screen sharing but I do it for other things. It goes something like this:
Allow one to SSH into your home computer via System Preferences -> Sharing -> Remote Login. I'd limit it to just your user if you use multiple accounts. I highly recommend setting up an SSH key and disabling remote password logins. That means that you can only SSH if you have the correct private key (it won't let you try to login by typing in your account's password). It makes it more secore. Google it.
You'll have to open up a port in your home router to let SSH traffic in. You can use pretty much whatever outside port you want but it should be routed to your home computer/server on port 22.
Now, it's just a matter of using the correct ssh command to do this. You want to use 'ssh's '-L' argument. Something like "ssh username@homeservernameoripadress -L 5900:homeservernameoripadress:5900". This allows port 5900 traffic to go through your SSH session all without opening up port 5900 in your home router/firewall. 5900 should be the port Apple use's for screen sharing. Once you've got your tunnel setup, you just access it like you would if you were at home but at "localhost" instead of your home computer's name/ip address.
If your home internet service provider uses dynamic ip addresses, you can use a free service like dyndns.org so you can have your own domain name so you don't always have to keep track of what your current home IP is.
Hope this helps. Google "ssh tunnel" for more info.