You can buy grit at the feed store. You can toss it on the ground for them to search and find, which is the natural way for chickens. Or put it in a container near their feeder. This is necessary for their gizzards to operate efficiently.
Chickens have no teeth because they don't need them. They swallow their food and it goes right into the crop. It gets churned up and broken down a bit, then the crop empties into the gizzard which is where the real grinding and digesting occurs. This is where most of the grit ends up. Grit takes the place of teeth. No grit, no digestion, and the chicken can develop a blockage when the food isn't ground up enough to pass out of the gizzard.
It's a nightmare trying to unblock a blocked gizzard, so it's far easier to make sure chickens have adequate grit.