Is your school blocking certain sites? Facebook is blocked, but you'd like to visit it? You wanna go to Facebook, but its blocked by the school IT department?
An unblocker is like a gateway that pass information between the computer you're on and the site you're visiting. The unblocker visits the site on your behalf, loads the page, and sends it on to you. So the site you're trying to visit only sees the unblocker's IP. So, basically you are browsing sites in an indirect way. School firewalls think you are just visiting the unblocker, not the destination site.
Another way of unblocking Facebook in school is to set up your own unblocker tool. This is harder than using someone else's unblocker but it's a lot nicer to have because you can make more of them. Explaining how to do this is not in this guide, do a search for "How to make your own proxy" you will find info. You can also make money from your unblocker by letting people advertise on it.
Other ways to unblock Facebook include to use sites that translate pages, like Google Translate or Babelfish. You can also try using Google's cache copy of the site. Unfortunately, you probably won't be able to to log in using these methods, so it's still best to use an unblocker for Facebook, however these tricks are good for most websites.