Salt rocks are salt shakers with a funnel underneath through which salt is added. When a filled salt rock sits upright, salt rests in the well created by the funnel. When the salt rock is shaken, salt is tossed out through the funnel.
Because dispensed quantities vary, I recommend shaking salt into your hand first, rather than directly onto food. Avoid moist salts (e.g. sea salts that retain some natural moisture); they tend to clump.