We commonly use this like, "She is really down to earth" or "She is a down to earth person." The inference is that a person's "head is [not] in the clouds;" they are mentally "grounded" or "on earth" as opposed to in the clouds, sky or in space. The humble concept of "down to earth" is the idea that you are not "above" everyone else (i.e. on a pedestal), you are "grounded" or stay on the same level as others. I threw several extra phrases in for additional context and learning. I hope it helps. Take care!