Actually, I meant to infer that in the dreams their limbs are bare and exposed, not covered up. The movie costumes are actually more conservative than in the dreams.
They are the personal guard of the royal family, particularly the King. One young woman is chosen from each tribe in Wakanda - usually the most beautiful and high ranking - and they are trained as warriors and as potential wives. So they are known as not only bodyguards and our country's best fighters, but as wives in training in case the King chooses one of them to be his Queen.
To be clear, I do not follow that tradition of choosing a wife from among them. There is already another woman in the dreams who has my heart.