Root is the term Linux uses for Administrator (Windows). Having Root access means you can remove Bloatware (like Sprint Football, Nascar, Moxier Mail, or basically any app that you don't want that is installed by Sprint but is installed in a "Root Only" section of the phone's OS.
I just Rooted my Moment last night. To do that, I had to Flash the phone with a custom Kernel from SDX. From there, you can manually remove applications from your phone by using a terminal program like Connectbot (available in Android Market).
Here's how I rooted my phone.
1. Downloaded the JoeyKrim 2.0.2 custom Kernel with Recovery.
2. Used the Sprint Updater tool for the CL14 release (the newest tool won't work without a hack) to flash the Custom Kernel to the phone.
3. I then created my own custom ROM from Joey's website. Doing this allowed me to add Live WallPapers (overrated), 3D Gallery (awesome), HQ YouTube (plays High Quality by default), and removed any pre-installed apps like Moxier Mail, Sprint stuff. I then loaded some custom boot images, like the Samsung/Android loading screen, and then the Nexus One Bootloader image (really cool). It then packaged it up into a nice neat .ZIP file which I saved to my SD card.
4. After saving the .ZIP custom ROM to my SD card, I rebooted the phone and launched recovery mode (power down the phone, then press and hold Vol. Down, Talk, End to launch recovery mode). I then chose "Apply .ZIP" and it loaded my custom ROM.
My phone is now fast, and even better than it was before.