The idea is to show what fraction the population of a county (in California) lives close to transit stops (within 1/2 mile/walking distance), broken down by ethnicities. One would expect that the white population lives further away from transit stops than latinos and African Americans given their respective socio-economic statuses.
Another way to think about it is that as counties develop, the transit stops are more dense in areas with non-white populations owing to high ridership. But, in a county like San Francisco, this is untrue, since it's a densely packed city with lots and lots of transit stops. Furthermore, there have been studies that have tied public transport to mental and physical health, happiness, and other human factors.
Also, availability of public transport stops also affects a county's population. This might help us understand how public transportation (nerve of a city) affects housing/settlements and how to design transportation systems in a more cost-effective, smart, and useful way.