Seedy toe is the hoof wall separating from the underlying structures at the toe. It can be caused by conformation (long toe and low heel), laminitis or simply poor quality hooves.
Not all horses with seedy toe are lame - and lameness often indicates infection.
It's very treatable, but treatment does involve removing the diseased horn, and has to be done by an expert. This is followed by special shoeing to support the foot until the hoof grows out properly, which can take a long time - think about how fast your fingernails grow. Hooves grow at about the same speed.