Standard deviation is a technical indicator that you might sometimes see shortened to StdDev. It basically measures the volatility of a currency by determining the size of the price fluctuations relative to the moving average. For example, if the indicator`s value is high, the currency's movement is considered to be volatile and you will see that bar prices are spread out away from the moving average. If the indicator value is low, then you can consider it as non volatile which means bar prices will be close to the moving average.

the most common way that traders use the standard deviation indicator as it is found on the MetaTrader platform is predicting reversals on the basis of the principle of reversion to the mean. Regression to the mean also underlies the principle on which oscillators like the RSI are constructed, and stipulates that each period of deviation from the average must be followed by a return to the same in such a way that the overall distribution of prices will fit the standard distribution.