The crop is the pouch on the front of the chicken where food goes right after they swallow it. You can feel the crop easily, because it just has a layer of skin over it.
In the evening the crop probaby feels like a big ball, because it's all full of food.
By morning the crop is usually empty, and it shrinks and becomes hard to find.
If you feel in the morning and the only thing there is a hard piece of wood, you can assume it's stuck, because if it was not stuck it would have passed on like everything else.
The reason for checking very early in the morning, before the chicken eats, is because it's pretty easy to tell if the crop is empty or not, but rather difficult to tell what is in there when it's full.
And if you aren't sure whether it feels normal or not, just grab a few other chickens and feel their crops for comparison.
