'Keep' in this case means to continue or stay in some condition. Keep well = stay in good health. Keep doing that = Continue to do that, do not stop doing it. "Keep healthy" = Stay healthy, remain in good health. In the other sense: "We are not there yet. We have to keep driving." = We have to drive some more. We cannot stop completely.

The words "retain" and "keep" are synonyms. I could say "retain good health", but people do not use that expression in English very much. I don't know why. 可以说保持健康吗？