Variance is a measure of how spread out numbers in a particular dataset are.  Higher variance measures mean that the data in that dataset are more spread out than than in datasets with lower variance measures, and a variance measure of zero would indicate that all of the numbers in a dataset were exactly the same. 
For some data sets, different types of diversity in our community result in measurement variance. For example, Forsyth County 11th grade students earned a wide range of scores on the ACT; some students scored close to the average score while others scored much higher or lower.  Variance also occurs in sample data, and high levels of variance among survey respondents can make survey estimates less precise.