Mothballs are made from a chemical called naphthalene which has the unusual property of subliming at room temperature. There are several chemicals that do this, like iodine crystals, dry ice and snow! What happens is that the molecules are not too tightly bound to one another and can leave the surface of the solid while not having to go through the liquid phase.

the definition of sublime

To convert (a solid substance) by heat into a vapor, which on cooling condenses again to solid form, without apparent liquefaction.

The word heat is a relative thing since dry ice sublimes at around -109F!