This word tends to baffle people because it can mean several things, though they all correlate to the built environment. As related to architectural exteriors, a soffit is the underside of a part or member of a building. This especially applies to an arch, but it could be a beam, cornice, balcony, or lintel. On the interior, this definition could also apply to the bottom of a staircase or an overhang.
A more modern application of the word soffit is a lowered ceiling plane around the perimeter of a room, above cabinetry, or any place one wants to place recessed lighting. Designers may also use soffits to delineate a circulation path or a change in space function. Soffits can be with constructed with metal or wood stud framing, and they are typically finished with drywall. See – they’re not so mysterious afterall.