The crop is like a storage tank that holds food the bird has eaten. It gets held there and released into the rest of the digestive tract as the food in the intestines gets digested. When the bird poops, it makes more room in the intestines and more food is released from the crop.
A crop should fully empty within 4 to 6 hours if things are digesting as they should. If the crop still has food in it on the morning, something is slowing it down, whether that be a crop issue, worms in the intestines slowing everything down, egg bound or the bird has some sort of illness that is slowing the crop down.