It refers to a flap on a desk that is pulled down to give a writing surface.

The flap of a bureau or secretaire that pulls forward to provide a writing surface.

The flap of a bureau or secrétaire that pulls down to provide a writing surface. Find home office furniture.

A drop front or drop lid, as in a cabinet-desk or piano. Sometimes "slant front".

A piece of furniture, such as a desk or piano, possessing a drop front or drop lid. See also Drop Front, Slant Front.