The pony i ride has one, its quite confusing but basically its when a horse is worked too hard too quickly or can be from being worked lots on hard ground. Its on the leg (cannon bone) and its because of how they evolved. They originally had toes (i think it was 3) so there was support there however through evolution the toes became one hoof, so this results in all the horses weight being spread over one relatively narrow point as opposed to 3toes. So if they are worked hard too soon before not warmed up the bone sort of divides, not quite as bad as it sounds but once they have it once they can be prone to it later if not correctly warmed up etc. There will be slight lump on the leg if you feel it. Hope this helps! Type it into google (get some good results) or ask your vet next time they come out