Standard deviation is a widely used measure of variability or diversity used in statistics. It shows how much variation or dispersion exists from the average (mean, or expected value). A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data points are spread out over a large range of values.
xi = Monthly fund return
x = Mean of monthly returns
n = Number of values
To annualise standard deviation, multiply by the square root of the number of observations per year e.g. quarterly will be √4 and monthly will be √12.