The short version, A tone outside the audible range of hearing of most people over 30. This means that you can get phone calls and receive text messages in class or school without teachers hearing it.
Follow the link for sample tones, but count me a skeptic. As a fuddy duddy 36 year old, they say I'm only supposed to hear the tones up to 14.9khz. As it turns out, though, I can hear every tone up to 22.4khz. Except, that is, for 15.8khz where there's either a hole in my hearing or my speakers.