Everything except air and sky is material, is mortal, is solid, is definite. The essence of that which is material, which is mortal, which is solid, which is definite is the sun that shines, for he is the essence of sat (the definite).

In plain words: Everything except the wind and the sky has a shape, will die, and is solid and fixed. The shining sun is the very essence of all these solid, mortal things.