Bollinger Bands are the most popular trading indicator created by John Bollinger.
These bands are commonly used to identify market volatility and potential overbought or oversold conditions. They are created by drawing two lines that are a certain number of standard deviations away from a moving average.
In simple term, Standard deviation means how much something varies or far away from its average line.
When the bands are narrow, it indicates that the market is experiencing low volatility.
When the bands are wide, it indicates that the market is experiencing high volatility.