We’ve learned though the years that lots of people don’t even know what a soffit is, let alone why they are so important. Let’s change that shall we?
A soffit is a piece material that goes from the very edge of your roof (touches the fascia at this end), to just past where your exterior walls meet the roofs support structure. They are on the underside of the roof. they have ventilation opening of some kind. This ventilation varies greatly and minimum amounts of ventilation are set in building codes. Most building codes demand metal soffits these days. A bit more expensive but they will last longer than you and as long as you have a good system of gutters to keep moisture away.
A soffits main purpose is to help provide air circulation in the attic. Since it’s also forcing an opening between the roof that top of the wall, it has to keep animals out of your attic (even flat roofs usually have an attic big enough for a whole neighborhood of rodent families).
Technically the word soffit means the underside of an architectural in a building. In modern America it’s almost always referring to the roof portion as described above.