This is a general term breeders and vets use to refer to any disorder of the limbs in a growing horse.
Horses come out of the womb with very long and rather fragile legs, and they grow rapidly. These problems are caused by genetics, nutrition (especially feeding a baby too much high energy food), and exercise.
The most prevalent is osteochondrosis, which is caused by feeding too much simple carbs (grain), mineral imbalances and trauma. It causes cysts and lesions on the cartilage, sometimes affecting growth. The animal may or may not be lame. Treatment involves restricting exercise and food intake both and sometimes supplementation. Surgery may be indicated.