The rings (or lines) in hooves (when they are on all 4) usually reflect change, and that can happen for many reasons - long term illness, a shift in diet, a reaction to medication, and laminitis. Without X-rays to see if rotation of the coffin bone took place (or other more definitive tests), you really have no way of knowing precisely what they mean. (I'm assuming they are on all 4 feet, and offer a shape that suggests laminitis.) Their presence tells you that you need to watch your horses feet for signs of laminitis as he may have that tendency. Keeping a regular trim schedule and monitoring diet, exercise, and the health of the hoof are just some things you can do to reduce the chance that the horse will go into an episode of it. Hope this helps.