Bollinger bands are a technical analysis tool invented by john bollinger in the 1980s. Bollinger bands are comprised of three bands which are referred to as the upper band, the lower band, and the center band. They are usually set to two standard deviations away (upper and lower) from the sma which forms the center band.

Bolllinger bands can identify periods of emerging volatility and can be used to identify potential changes in price as well as identifying possible break out trades, as the price moves towards the outer bands as per the diagram below. Using bollinger bands like this makes it very easy to visually identify those periods when the market is more likely to breakout in the short term. Typically before a break out, the bands can be observered narrowing towards the center band sharply.  A characteristic of  bollinger bands is that price will always tend to return to the middle of the bands. 

                                                                                  Bollinger Band Break Out