The standard deviation measures how spread out a set of data is around the average (mean) value.
Normally, about two-thirds of the values in a set of data will fall within one standard deviation above or below the average, and only one in 20 will fall more than two standard deviations above or below the average.

A very low standard deviation indicates that the majority of the values are close to the mean (there is little spread), thus the mean is a good indicator for the sample as a whole. Conversely, when there is a large standard deviation there is a lot of spread and the value of the mean as an indicator is reduced, as any value may be far from the mean.