Otherwise known as "seedy toe," white line disease is caused when the hoof wall separates from the sole at the toe. This tends to cause an infection between the hoof layers that damages the hoof and may or may not make the horse lame.
Some horses are prone to getting white line disease - otherwise, it's generally caused by a hoof injury that was not detected at the time.
It's treated by opening the bottom of the hoof wall and cleaning the area thoroughly, then applying a special shoe temporarily. Some horses can continue in work, others may be lame until the hoof grows out (which can take up to 8 weeks).