(noun) - A remarque is a small, personalized drawing or symbol that an artist adds (near his or her signature) on a print. The presence of a remarque increases the print's value.
Originally, remarques were remarks made, in pencil, that identified the various stages a printing plate went through while in the process of being finalized. Whistler seems to have pioneered making remarques desirable to collectors, particularly in the case of his "butterfly" mark.
Nowadays, a remarque is primarily a "value-added" option for print collectors - meaning: You can have an artist's remarque added to your graphic edition, if you care to shell out some more money. (Unless, of course, the artist is a personal friend, or owes you a favor.)