What Is Variance?
Variance measures how spread out a data set is by averaging the squared distance of each value from the mean. Squaring makes every difference positive and weights larger deviations more heavily. Its square root is the standard deviation, which is usually easier to interpret since it's back in the same units as the original data.
Example
For the data set 2, 4, 6: the mean is 4. The squared differences are (2−4)²=4, (4−4)²=0, (6−4)²=4. The population variance is the average of those: (4+0+4)/3 = 2.67.