Grit is insoluble and has no nutritional value for your flock, but is an important aid in helping chickens digest their food. Without teeth, food travels to their gizzard where it is digested along with small insoluble materials, like rocks, to break the food down into a digestible paste your chicken's body can absorb.
Insufficient grit, either foraged for naturally or provided free choice, can result in things like crop impaction and sour crop.
When Should You Provide Your Flock Grit?
Common practice is to provide grit to your flock when they begin to eat food other than their regular feed. This can include things like treats, mealworms, grasses, and other foraged finds.
Grit is available in various sizes to accommodate the age of your flock and various poultry types. For instance, turkeys use much larger grit than baby chickens to digest their food.
If your flock free ranges, they typically find natural grit like rocks and sand suitable in helping their digestion. However, soil types and environments differ, so it's best to provide grit as a free choice for your flock.