su will give you root priviliges to do certain things you can't do as a normal user. this is usually for installing, removing software, editing certain configuration files and stuff like that. it's a safety thing so you don't accidently destroy your system by typing something a user ought not type. it's also a safety thing so if you get hacked, they can only attack your user profile and not do any real damage to your system.
you might wanna check out rute
...it's an excellent online tutorial.