Sand toe is a sprain of the top side of the first metatarso-phalangeal joint; the joint that joins your big toe to the rest of your foot.  The injury occurs when the toe is hyper-flexed, usually from being dragged forward along the ground.  This is a common problem in beach volleyball players since they are playing on such an unstable surface and making quick changes in direction.  It also is seen in martial arts athletes since they are barefoot and making quick changes in direction and grappling. 

Proper rehabilitation of the sprain is necessary to prevent any permanent dysfunction.