Horses can loose large amounts of essential body salts through sweating due to exercise or during hot and humid weather which could lead to dehydration. Salt contains two of the most important electrolyte substances that balance fluid in the body, sodium and chloride (potassium being the third ) so it's important to make sure your horse has access to adequate amounts of salt to restore the balance.
Some equine nutritionists advise adding one or two tablespoons of salt per day to your horse's feed while others recommend free access to salt via a salt lick in the stable or field. Most equines will actively seek out salt and self-supplement, generally there is no need to worry about them taking too much salt.