"Down to earth" usually describes someone who is famous, or has a powerful job, or lots of money, or has some impressive qualities BUT lives his/her life just like a normal person. Even though they have wealth, power, or amazing talents, it's easy for regular people to relate to them. They speak the same way we do, they dress like a regular person, etc. Does that make sense?

a down to earth person = a practical, hands on person.Someone who gets things done.

it is not slang....it's a common expression (adjective). Adding more examples to the excellent answers above: - Though he was chosen as the new team leader in the sales department, he asked us what he should improve in his potential and for more advice, he is a down to earth ideal person. - I like Christina, because she is so down to earth. I saw her talking to an old lady-beggar in the street the other day. When i asked her about that, she told me that she invited the lady home for a warm dinner and clothes.

It means 'being modest'!

To add more into this discussion, there is a close association between being down to earth with not being too much exclusive as if you were a freaking-some special creature from another world/planet (this is why it's called "down to earth"). Such exclusiveness may make that individual/group estranged from what common people perceive about life or the subject matter.