The waxy coating on leaves is called the cuticle. The cuticle lets water roll off the surface of the leaves and helps protect the plant from disease.
Sometimes when rain storms move in, especially if the weather has been unusually dry, tree leaves flip upside down so the part of the leaves not covered in cuticle faces up. This allows the rain to soak into the porous underside of leaves. When leaves are flipped, in addition to the pores of the leaves, the veins that carry nutrients and water inside the leaf are exposed. Leaves then use carbon dioxide, water and chlorophyll to carry out photosynthesis.