To 'come back down to earth' means that the person returns to cold hard reality after a period of daydreaming or excitement.
To 'bring someone down to earth' is to cause someone to return to cold hard reality after a period of daydreaming or excitement.
A person described as being 'down to earth' is therefore someone who does not go off in his mind on flights of fancy, daydreaming, imagining. He sticks to the matter-of-fact here and now. He's what we term ' a sensible person' and 'someone with a good head on his shoulders'.
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