well there are so many threads about jailbreaking it isnt funny so searchbar it.
second off the reason to do it: it installs an icon onto your ipod that is an app so you can download wallpapers, games, and a bunch of generic apps that prolly dont work as well as the ones itunes offer. but they are for the most part free.
it all depends on what kind of touch you have if its a g1 then you will prolly have an easier time doing it, look it up on google, its really not that hard you just have to PAY ATTENTION to the steps they provide.