Salt absorbs water from the air, which can cause it to clump.
Salt is naturally hygroscopic, which means it absorbs water from the air. When grains of salt absorb enough moisture, they dissolve slightly and stick to each other.
Many refined salts add anti-caking or flow agents to prevent this natural clumping reaction, but Real Salt does not have these additives. The clumps usually break up pretty easily unless they’ve been exposed to a lot of moisture.
Adding uncooked rice to your salt shaker can help absorb moisture and keep the salt from clumping up. In most shakers, the salt will come out but the rice is too big to fit through the holes.
Here are some interesting articles about clumping salt:
And here's a blog post with helpful storage tips that can keep your salt clump-free: