Clapper: a pressing tool made of wood specifically helpful in tailoring techniques. A clapper is used to press things flat.
With a hot iron, shoot a bunch of steam onto an area of the item to be pressed (usually a seam, collar or other sort of edge). Immediately place the clapper on top of the area just steamed and hold there for 20-30 seconds or so. The clapper holds in the heat and moisture and helps the fabric to stay flat.
*Note: the tool pictured above is a clapper and a point presser combined. The clapper is the bottom portion of the tool--basicially just a piece of wood.