Dead or infected material leaving the inside of the hoof. The most important thing you can do in this situation is to trim in order to restore proper hoof bio dynamics and keep the hoof clean and protected while it heals. Soaking in Epsom Salts is recommended and will help to draw out the material faster.
I have special soft boots the horse can wear and have developed a healing mud pack for this purpose. Once the abscess has sealed enough to kept free of infection, I recommend casting the hoof for a period of time appropriate to your specific situation. To continue to promote healthy hooves, a strict diet and trimming schedule must be adhered to. Please inquire about the abscess kits I keep in stock.