I believe it's de-ionized water. Maybe it's the same as Boog is talking about. I assume they use a low amount of it due to the relatively high cost of "d-i" water vs tap or soft water.

Pure d-i water will dry on glass and leave absolutely no spots. It doesn't taste good to drink, because it's tasteless. I believe I've also read warnings against drinking d-i water since it can deplete the body of minerals, somehow. But, you normally find it in car washes. I think it's also preferred for using in radiators & batteries.