Oscillator

An oscillator is an indicator that lives in a box under your chart and swings between two extremes, like a pendulum. The RSI and the stochastic move between 0 and 100; others, like MACD, swing around a zero line.

Near the top of its range, an oscillator suggests price has run hard and may be stretched ("overbought"). Near the bottom, "oversold". They shine in sideways markets and can mislead badly in strong trends, where overbought can stay overbought for a long time.