A fine evenly woven linen or cotton cloth that is wetted and placed over the area you want to press, beneath a very hot iron. The steam generated is forced directly into the fabric beneath, which embeds the stitching and sets the seam or dart end.
For woollen and worsted cloths steam is essential to press seams flat, to shrink the cloth where necessary and to create the best fit along the stitched line, and a pressing cloth is the best delivery method.
The pressing cloth, whether wet or dry creates a protective space between the iron and the fabric being pressed; this is especially useful for all delicate fabrics that might show iron marks or damage easily.