This is the percent of the total population that self-reported their race in the American Community Survey as black or African American.
The census racial categories are: White alone, Black or African American alone, American Indian and Alaska Native alone, Asian alone, Native Hawaiian and Other Pacific Islander alone, Some other race alone, Two or more races. Note that there is also reporting on ethnicity, which the census defines as either Hispanic / Latino or not Hispanic / Latino.
Demographic data is a critical component to any discussion of health equity. In talking equity, or inequity, we are comparing such things as age, race, and economic standing to determine whether there are inequities between different demographic groups.
Ethnic and racial composition measures the ratios of whites, blacks, Hispanics, and other groups in a neighborhood. These numbers are significant because health care inequity and the social conditions that pertain to it tend to cluster in regions with high minority populations.