Worn in combination with the dry layer, its function is to produce a warm layer of insulation. Traditonal materials for insulating layers are wool, pile or down. Now synthetic fabrics are often chosen because they're made to be light-weight as well as insulating. Each of these materials provide space for dead air which is warmed by the body to produce a layer of insulation. The denser a material, the faster heat can be transfered by conduction.
Air is much less dense than a piece of fabric, so less heat is conducted through a layer of air.
It's not the clothing that keeps you warm, it's the dead air trapped by the clothing. Polartec is a synthetic material made of polyester fibres that have been brushed on both sides. The brushing produces tiny pockets that trap air which is then warmed by your body to produce a layer of insulation. Along with providing insulation, the polyester fibres also help to pull moisture to the surface for quick evaporation. Polartec fleece is lightweight, quick drying and compactable.