Oscillators measure the relative strength of recent price moves. 

Buy a value between 0-100 hands their name, if prices rise strongly - an oscillator will follow and reach overbought levels and give you a sell signal.

Similarly if the prices are falling - an oscillator will reach an oversold level and give you a buy signal.

Oscillators work well in ranging markets, but lead to whipsaws when markets are trending.

Popular oscillators are: RSI , stochastic and momentum.