A French term from the Italian basso-relievo ("low relief"), bas relief is a sculpture technique in which figures and/or other design elements are just barely more prominent than the (overall flat) background.
Bas relief is created either by carving away material (wood, stone, ivory, jade, etc.) or adding material to the top of an otherwise smooth surface (say, strips of clay to stone). This is a technique as old as humankind's artistic explorations, and is closely related to high relief.
As an example, here you can see one of Lorenzo Ghiberti's (Italian, 1378-1455) panels from the East Doors (commonly known as the "Gates of Paradise," thanks to a quote attributed to Michelangelo) of the Baptistery of San Giovanni. Florence, Italy. To create the bas relief Creation of Adam and Eve, ca. 1435, Ghiberti first carved his design on a thick sheet of wax. He then fitted this with a covering of wet plaster that, once it had dried and the original wax had been melted out, made a fireproof mold into which liquid alloy was poured to recreate his bas relief sculpture in bronze.
"Through all of this turmoil the Shaw monument had come to bear more and more upon my mind, and finally reached the long looked-for day of its unveiling. There had been much good natured abuse of me for the time expended on the bas-relief." - the sculptor Augustus Saint-Gaudens (American, 1848-1907) on his Shaw Memorial. From his autobiography The Reminiscences of Augustus Saint-Gaudens.