A chicken's crop is a part of her digestive system, and is located on her breast. You see it in the illustration below, closest to the front. When it fills up with food, it may stick out a little bit.
It will be smaller in the morning before she has eaten, and larger after she is full. The crop or craw holds food before it goes further down their digestive tract. Chickens basically store food in their crops; it is like a chicken lunchbox. Then it moves into the stomach or proventriculus and gets soaked with digestive juices, before finally getting "chewed" in the gizzard.
The gizzard is a muscular part of the digestive system that "chews" food by using small stones, or grit, to grind the food up. Chickens need grit because they don't have teeth. Chick starter is typically milled finely enough so that baby chicks don't usually need grit unless they're eating something other than starter. Chickens will need grit once they begin eating treats and other foods, for example if they're foraging in the yard. If your chickens free range, they will probably pick up grit, gravel or small stones on their own, but we usually recommend providing extra just in case.