In the 1983 movie "Wargames" starring Matthew Broderick, the hero is a young computer hacker who programs his computer equipped with a modem to dial every possible phone number combination looking for another computer to answer in an early form of digital exploration. Wardialing was never all that productive as a means of discovering other computer systems because it took a very long time to cycle through enough numbers to find much and if you did the chances were relatively slim that you would be able to do anything interesting with the computer on the other end of the line.
But times have changed considerably and new technology has allowed this old idea to be reincarnated into something analagous to the technique of wardialing. Wireless access points have become very popular in the last couple of years as they have gotten to be relatively inexpensive. These wireless access points usually broadcast far beyond the confines of the small office or home office in which they are set up and are often set up by people with no knowledge of security or how wireless networking actually works. They simply want the convenience of not having to be tethered by a wire all the time.
War Driving is also called 'whacking'.
Much like the kid with the modem searching for other computers with modems, a person with a laptop with a wireless network card can simply power up their computer and their wireless card will try to communicate with any nearby wireless access point. If they happen to be closer to someone elses wireless access point than their own they could end up talking to that one. You can drive around town with such a setup and your computer will detect any wireless access point it happens to come within signal range of. This is known as wardriving. If you do it from an airplane, it is warflying. This name might seem to connote some sort of violence but it is purely derived from a movie title and is in no way actually physically threatening.