It gives you access to the phones root folder, so you can do with it pretty much whatever you want, first of all it unlocks multitouch so you can zoom like the iphone does by pinching. You can do other things like overclock the phone, run different ROMS and themes. Seen the new HTC Hero? You can run its version of android, or like me, run a theme by using the Android Developers Phone ROM that makes it look like the hero, and use the music and camera app from the Hero. You can also take screenshots of your phone.
This explains it:
How to do it:
This is what I'm running
http://forum.xda-developers.com/showthread.php?t=537684