What are we measuring?
The Gini index is the most commonly used measure of income distribution and is estimated annually for U.S. populations by the Census Bureau based on household income. The Gini index has a theoretical range of 0 to 1. An index of 0 indicates perfect equality, where all households have the same income. An index of 1 indicates maximal inequality, where one household has all the income and the others have none. The Gini indices for the U.S. and for the state of Georgia for 2016 were both around 0.48. (It should be noted that the Gini index can be calculated in different ways and can therefore vary between sources for the same population. The Census Bureau, for example, calculates the index based on pre-tax income, so their index will appear high compared to sources that use income after tax transfers.)
Why are we measuring it?
While there is some debate over the effects of income inequality on society, most economists and related experts agree that growing income inequality can have a number of undesirable consequences. Among these are slowed economic growth, increased likelihood of economic crisis, lower income mobility over a lifetime and between generations, increased poverty rate, and political polarization.
How are we doing?
The Gini index for Fulton County was 0.55 in 2017, higher than that for the U.S., Georgia, the Atlanta metro area, and all of its national benchmark counties.  The Gini index for the U.S. was 0.48 in 2017.
 Income Distribution in Counties with the Highest and Lowest Gini Index