If you have ever looked closely at a the surface of a horse's hoof you will notice that they tend to have rings that run round the hoof, some which are somewhat raised and wide and some that are less pronounced and narrow. These rings are indicators of the growth from the cornet or top of the hoof and indicate any number of issues, much like the rings on the inside of a tree mark periods of growth.
A foal's hooves will be very smooth, simply because they have had a consistent environment when they were carried by the mare. Once the foal is out on pasture he or she will go through periods where they are stressed, where they have lots to eat, and where they are experiencing lots of exercise and movement or very little. Each of those factors will affect the ring on the hoof and will leave either a raised or flat ring, or a wide or narrow ring.
Basically the ribs or rings are normal and are not any sign of problems with the horse, unless they are abnormally thick or pronounced. In some cases where blood supply is inadequate to the hoof the ring pattern will be abnormal, giving the farrier or the vet a clue that circulatory problems may be present. In many horses these rings are known as "grass rings" and will change based on the condition of the pasture and if they are fed supplements or not. In poorly nourished or starved horses the rings will be very small and close together as the body did not have the nutrients to produce normal hoof growth during periods of starvation. These rings are again normal and should be evenly spaced around the hoof and parallel to the ground.
Often horses that have had trouble with laminitis will have a ringed or ribbed appearance to the hoof. Usually these rings will be very uneven and the rings will be closer together at the toe and wider towards the heel. In this case the rings may indicate a problem with the possible separation of the hoof wall from the laminae, which could lead to founder. This is often a good indication about any possible problems with laminitis in the horse's recent past.
Treatment for ringed or ribbed hooves is really in balancing the horse's feed and keeping him or her healthy as possible. As the hoof grows the rings or ribs will be removed by the farrier through routine trimming and filing. If you are concerned about the rings a hoof conditioner can be used to help maintain the hoof, but it is usually not necessary. Never try to file or sand down the rings as this will remove the natural protection from the surface of the hoof and lead to dry, brittle cracked hooves.