If you want some more details, check out the Wikipedia site on the process:
http://en.wikipedia.org/wiki/Rooting_(Android_OS)
Look at it this way. When you get a Windows machine, you can install programs that do basically whatever you want to the operating system. You can browse around and experiment with different features. You can fool around with system files and change various settings. On an Android device, the OS basically prevents you from doing these things, mostly for your own protection--if you don't know what you are doing, you can completely mess things up.
"Rooting" is basically the process of unlocking your device so you can mess around with it like a Windows machine. For owners of most Android devices, I don't see any need to go through this process; they work great right out of the box. In the case of the eDGe, though, the operating system has a number of bugs, and rooting allows you to do a number of things that might improve your experience; for example, without rooting, it's impossible to install any of the Google Applications (gapps) such as Gmail, Google Maps, and the Android Market.