Stocking up is swelling in a horse's lower legs. It's caused by fluid retention and is most often seen if you have to put a horse on stall rest. Some horses are more prone to it than others, and it's also a problem that increases with age.
The treatment is just to get the horse moving - the problem usually fixes itself after about half an hour of light exercise and some people swear by hosing the legs with cold water. The swelling is usually in all four legs or one pair (unilateral swelling often means something more serious).