Bollinger Band is a forward looking Volatility Indicator. It has 3 components to it. The central average line, upper band and lower band. The central line is the average line. If you have selected 30days, then the central line is the last 30 days moving average line.
The upper band is 1 standard deviation higher than the average and the lower band is 1 standard deviation below the average.
BB works on the premise that stocks prices over a long term reverts to mean. So if the stock price is at the higher band (1 standard deviation away) then there could a 66% probability that it can revert to the average line.
Likewise, if the stock price is at the lower band, there is a 66% probability that it comes back to the average line.
In simpler terms, you short the market/stock when the stock price hits the upper band with a target the price as reflected on the central line. If the price is at the lower band, you buy with a target as reflected by the central line.
BB is one of the better quality indicator as it has some tangible math behind. You can improve the efficiency from 66% t0 95% buy tweaking few parameters in your BB settings.
Happy Trading.