SuperUser is an app that lets your device have more permission than it did before it was rooted. It allows you to use all sorts of apps that can only be accessed when rooted. The easiest way to root IMO is to use Universal Androot. Its a free app, but you'll have to find it on the internet as it is not on the App Market.
Basis is correct on his description on Super User access.
on a side note though, if you do decide to root you phone please read the sticky about rooting in the hacking section. there are a lot of positive as well as possible negative things that come out of being rooted. i would highly recommend doing some research and also learn how to unroot in case you need to do so for whatever reason (trust me it will put you at ease if something does go wrong)
