Standard Deviation — value of the market volatility measurement. This indicator describes the range of price fluctuations relative to simple moving average. So, if the value of this indicator is high, the market is volatile, and prices of bars are rather spread relative to the moving average. If the indicator value is low, the market can described as having a low volatility, and prices of bars are rather close to the moving average.
StdDev = SQRT (SUM (CLOSE – SMA (CLOSE, N), N)^2)/N
SQRT — square root;
SUM (…, N) — sum within N periods;
SMA (…, N) — simple moving average having the period of N;
N — calculation period.