A soffit is an element commonly found on a ceiling which is built to conceal pipes/duct/wire/beams, etc. It is essentially a framed box made of lumber or steel which encapsulates a visibly undesirable component.
A soffit will later receive finish materials like drywall or plaster. Soffits may also be utilized as a decorative feature such as above cabinets or on vaulted/trayed ceilings.
2×4 lumber is commonly used. A soffit may run along a rooms walls or be independent. Two (or more if particularly wide) ‘box-frames’ are typically built first at the appropriate dimensions then attached to the wall and/or joist. Lateral framing members are often installed every 12/16/24″ On-Center for additional support.