Bollinger Bands is a technical analysis tool used to measure volatility in the stock market. It consists of three lines drawn on a chart, including a middle line that represents the average price and two outer lines that represent levels of high and low prices. The distance between these lines indicates the level of volatility or uncertainty in the stock's price movement. When the Bollinger Bands contract, it suggests that the security is becoming less volatile, while when they expand, it signals an increase in volatility. This tool can help investors make informed decisions about buying and selling securities by providing insight into potential price movements based on historical data.