This trend indicator is known as the middle band. Most stock charting applications use a 20-period moving average for the default settings. The upper and lower bands are then a measure of volatility to the upside and downside. They are calculated as two standard deviations from the middle band
As Bollinger puts it, moves that touch or exceed the bands are not signals, but rather “tags”. On the face of it, a move to the upper band shows strength, while a sharp move to the lower band shows weakness. Momentum oscillators work much the same way. Overbought is not necessarily bullish.

Bollinger Bands – Conclusion
In conclusion, the Bollinger bands indicator is a versatile technical analysis indicator. This indicator allows traders to gauge both the trend and the volatility on the price chart. Formed as an overlay, the Bollinger bands are best used to gauge when to enter into a trend as volatility is just starting to increase.

Traders can also use many other technical strategies along with Bollinger bands and build a profitable trading system.Source: “How to read and use Bollinger bands”