Pressure and heat on a flat surface smooths the fabric and produces creases where needed. The key weapon in banishing creases in some materials is moisture. It penetrates the fibers and sames them swell, loosening the grip between them. While modern steam irons deliver moist air directly to garments, it used to be necessary to sprinkle water on before the hot iron was applied. After selling, the fabric fibers are dried by the hot iron as it smooths it, fixing the fibers in the desired position.
Modern steam irons provide all that it necessary for achieving smooth fabrics and perfect creases: heat, moisture and pressure.