Standard deviation measures the dispersion of returns around the mean. Higher standard deviation means more volatility — returns swing more widely above and below the average.
σ = √[Σ(Ri - R̄)² / (n-1)] where Ri = individual returns, R̄ = mean return, n = number of periods
If a portfolio has 10% average annual return with 15% standard deviation, roughly 68% of annual returns fall between -5% and +25% (one standard deviation from the mean).
AllInvestView calculates annualised standard deviation on your portfolio analytics page. Related: Sharpe Ratio, Maximum Drawdown.