means A strip of shaped material—wood, plaster, or otherwise—run along the joints and edges of a room or piece of furniture to trim, frame, and finish it.

from From the verb 'mould' (British) or 'mold' (American), meaning to shape or form, which traces back through Old French 'modle' to Latin 'modulus,' a 'measure' or 'pattern'—the same root that gives us 'module' and 'model.' The sense moved from the act of shaping material in a form to the shaped piece itself: a length of trim cast or carved to a fixed profile, repeated down a wall.