For instance, if we are studying cancer rates among census tracts in a given city local clusters in the rates mean that there are areas that have higher or lower rates than is to be expected by chance alone; that is, the values occurring are above or below those of a random distribution in space.
To put it another way, studying cancer rates within specific geographic areas can reveal local clusters where rates are significantly higher or lower than what would be expected by random variation; essentially, these localized patterns indicate deviations from the expected distribution of rates in space.