Overselling is a term used by hosts to sell more then what resources they have left. If you upload 100GB worth of files, your host will time out, and your site will be terminated for server resources. See, 100 other people have the same plan as you, and they're on the same server. If everyone tried to upload at once, the server fails, and everyone is terminated. Go Daddy is especially famous for that.
BTW, when will you buy a legal version of your BB software?