Bollinger Bands® are a kind of trading envelope. Bollinger Bands® consist of a moving average and two standard deviations charted as one line above and one line below the moving average. The line above is two standard deviations added to the moving average. The line below is two standard deviations subtracted from the moving average.
Traders generally use them to determine overbought and oversold zones, to confirm divergences between prices and indicators, and to project price targets. The wider the bands are, the greater the volatility is. The narrower the bands are, the lesser the volatility is. The moving average is calculated on the close.