As far as an explanation goes, "iPod for books" is probably a very good starting point. If they don't know what an iPod is, they'll probably decide they don't need an ebook reader, which (at this point) is probably a good bet. If they know enough to ask more questions after that, it's worth providing more info, including that it's often easiest to buy books from the same place you get your ebook reader, so they'll "work right." Anyone who asks "why" can be given the full info-dump.
