Bollinger Bands are a volatility indicator that are used to measure the current price of a security relative to previous trading periods.
In calculating a Bollinger Band the 'period' (number of days, weeks, months) must be defined in order to calculate a moving average from which is defined
Stockopedia's chart package default uses a 20 period Bollinger Band with a 2 Standard Deviation Width.
A full definition of Bollinger Bands is available at Wikipedia