Variance is a measure of dispersion. It is the average squared distance between the mean and each item in the population or in the sample.
An advantage of variance (as compared to the related measure of dispersion - the standard deviation) is that the variance of a sum of independent random variables is equal to the sum of their variances.
Note: When using the sample variance to estimate the population variance, the divisor (n-1) is typically used instead of (n) to calculate the average. The latter results in a biased estimate; the former is unbiased.