I am considering buying a Tivo series II from Amazon because 1) I can't get one at the Tivo site, 2) We have Satellite (no cable available in the sticks) and will have to use a phone line. Under the description of this particular Series II, it says: Everything required for installation is supplied; subscription to TiVo service is additional requirement. An appropriate set top box is needed for digital cable, or satellite. What is a set top box? Thank you.