This expression is derived from the Italian basso rilievo, the literal translation meaning "low contrast" as opposed to "alto rilievo" ("high contrast") and "haut-relief" ("high relief") in French.
"Relief" simply means a sculpture is on a flat surface and not "in the round." You can't walk around a relief because it doesn't have a back. High relief means that the forms are carved quite deeply and cast strong shadows. Low relief is smoother, flatter and shows less contrast of lit and shadowed areas.
Beautiful works of art resulted from the painstaking labors of artists such as Lorenzo Ghiberti. His bas-relief Creation of Adam and Eve was done in bronze. He first carved his figures into a sheet of wax which was then covered with plaster.