Literally, it means to keep the glittering casket and give back the pearls to the seller, well this is just a idiom that shows youur lack of judgment and end up choosing the wrong thing

one man think the wrappage is more beatuiful than the goods. so he buy the wrappage and throw away the goods. it really means, don't pay attention to appearance,we should distinguish the really cimelia.

"椟"is more precious than "珠",so if someone just buy "椟" and return the "珠" which inside the "珠", he is a stupid person who cannot see the true value of thing.

to buy a box but return the pearl in it