In statistics and probability, variance measures how far a data set is spread out from the average value.
Let X be a random variable, the expression for the variance is :
Var(X) = V(X) = E [(X-E[X]^2]
Variance can be considered as the average of the square deviation of the observations from the mean.
A distribution with a higher variance will be more widespread.
The variance is always positive or zero. when it is equal to zero it means that the random variable is a constant.