Grit is a general term for the small bits of rock, sand, shell, and other things birds eat to help them digest their food. These small pieces of hard material go into their gizzard, helping to smash up the seeds and berries that the birds eat. It rarely comes out of the bird, so a bird accumulates this grit over its lifetime.
Birds like doves and gallinules are most likely to use this technique. It was thought for a while that smaller birds, like parakeets, needed grit to digest their food, but it has since been found that they do not.
One threat to birds is that they will find lead shot pellets (from a hunter´s shotgun) on the ground and eat those, and the lead in the shot will slowly poison the bird. For this reason lead shot is being outlawed in many areas.