In a general sense, it refers to using someone else's technology. Specifically, it means to hop on board someone else's unsecured wireless network in order to access the Internet with your laptop or mobile device. For example, "let me stop in that hotel lobby, I may be able to piggyback onto their network without having to pay for the Starbucks hotspot." Another example, "At the moment, Google and Yahoo are not charging consumers to piggyback onto their mapping engines."
NetLingo Classification: Online Jargon