A Bollinger Band is an Indicator of Volatility, and the technical analysis tool is defined by a set of lines plotted by standard deviations, one positively and other negatively, and away from a simple moving average (SMA) of the security's price.
When the markets become more volatile the bands widen.
When the markets become less volatile the bands contract.
When the markets become more volatile the bands widen.
When the markets become less volatile the bands contract.