Birds like chickens need to keep small stones in their gizzards to help grind up their food. The crop is actually a different part of their digestive system; it's an expandable pouch that you can sometimes see and feel on the right side of your chicken's chest. Think of it as kind of a grocery bag that they can use to store food before beginning to digest it.
There are two kinds of grit: soluble, and insoluble. Insoluble grit is usually small granite stones or chips. This is the kind the bird stores in its gizzard for grinding up food.
Insoluble grit would be oyster shell. This kind of grit would not be useful to help grind up food because it dissolves in the bird's digestive system. Many people offer oyster shell as a calcium supplement to help their hens develop strong eggshells. However, you should not supply only oyster shell to your birds without supplying insoluble grit, too.
