This is the A/D oscillator, design by Jim Waters and Larry Williams. As any other oscillator, it measures the strength of the buy and sell pressure.
If a candle opens at the low, and closes at it's high, the signal given by the oscillator is 100. If it opens at high and closes at low, the signal will be 0. Since it completely disregard other values, it is common to use a smoothed version of it, which is the average of the last n values, just like the stochastic oscillator.
Tho it doesn't have much use for positioners or swing traders, day traders and scalpers can use it in confluence with volume to enter in a counter trend trade.