A soffit is a piece of material that is typically fitted between the wall of a building and the overhang of the roof. It both protects the roofing system and improves the appearance of the underside. Soffits are also used in porches, under arches or under stairs.
Soffits are a visible part of the roof structure. They are usually made from aluminum and vinyl, but they come in other forms, as well.
They have an aesthetic purpose, as they provide a finish to the exterior of a building. They are available in different colors, and some can be painted. Without them, the beams and interior of the roof would be visible.
Soffits also protect the roof, as they act as a barrier between the roof beams and structure, and provide protection from the elements. Without them, mold gets into buildings, and wood can rot.
Most soffit systems have ventilation built in, either as part of the board, or on top of it. This is important, as ventilation keeps roof areas cooler in the summer and prevents condensation from building up in the winter. Condensation leads to mold and rotting of wood. Ultimately, roof beams can be damaged without proper soffits and proper ventilation.