George Lane's construction, %K, measures the close relative to the highest high and lowest low of the last N bars. A second line, %D, is a short moving average of %K used as a signal-line. Readings above 80 are conventionally called overbought and below 20 oversold.
The underlying observation is reasonable: in an uptrend, closes cluster near the top of the range, and a shift toward closing near the lows can indicate fading strength.
In practice it saturates. During a strong trend it can sit above 80 for weeks, and every threshold-based short signal loses. It is far more defensible as a pullback timing tool within a trend you have already identified than as a standalone reversal indicator.
Related: slow-stochastic, stochastic-rsi, williams-percent-r, overbought, signal-line