Cropping is used by bees to temporarly store nectar of flowers. When bees "suck" nectar, it is stored in their crop. They can then fly while transporting it.
In a bird's digestive system, the crop is an expanded, muscular pouch near the throat. It is a part of the digestive tract, essentially a part of the esophagus. In adult birds, the crop can produce crop milk to feed newly hatched birds, however, not all birds have a crop. As with most other organisms that has a crop, the crop is used to temporarily store food.