This photo shows the underside of a horse's hoof. Horses hooves are generally very tough but even the slightest injury to the hoof wall (outer rim), white line (line just inside the rim), sole large flat part, or frog (v shaped part at the back of the hoof) can be painful and dangerous to the horse. The horse is designed to distribute weight evenly through their four hooves and a hoof that cannot bear weight puts a dangerously large amount of weight on the other hooves. In this photo you can see portions of the sole flaking off. This is a part of the natural exfoliation of the hoof and nothing to worry about.
The mare in this photo was suffering from an abcess which had not yet surfaced. Abcesses in horse hooves are tiny infections, pockets of puss, which can be very painful but will eventually work their way to the surface and drain.