This map is a little tricky to explain succinctly, so bear with me. What is being shown is the racial/ethnic group in each county that is most disproportionate to the makeup of the United States.
For example, my home of Travis County, Texas, is 50% White, 34% Hispanic, 8% Black, 6% Asian-Pacific Islander and 0% Native American. The United States as a whole is 62% White, 17% Hispanic, 12% Black, 5% Asian-Pacific Islander and 1% Native American. While Travis County is less White, Black and Native American than the US as a whole, it has a higher share of Hispanic and Asian-Pacific Islander residents.
The share of the population which is Hispanic is 2.8x that of the US, and the Asian-Pacific Islander population is 1.2x. Therefore, Travis County is shown on this map as Hispanic, even though Whites constitute the largest racial/ethnic group within the county.