Neat and clean in the careful, everyday way that takes time in front of a mirror — hair cut and tidy, nails clean, skin looked after, clothes pressed, nothing untidy anywhere. It is about the body and the upkeep, not the clothes themselves, which is exactly what separates it from well-dressed: an expensive suit on a man who has not shaved is not well-groomed. The word implies effort and habit; it is a small compliment that says 'this person takes care of themselves.' It comes out of the stable — to groom a horse is to brush and clean it — and the animal sense is still fully alive today: a well-groomed dog has been washed and brushed. That older meaning is worth knowing because it shows why the word focuses on brushing and cleaning rather than style. It is used about both men and women, though it is slightly more often said of men, and it sounds a bit formal, the kind of thing you would find in a description of someone or a note about job interviews.