Fascia is the term for any horizontal piece that lies atop a column or across the top of a wall. When you hear fascia referred to in the context of roofing it is referring to the board that caps the rafters outside of a building. The fascia can support a rain gutter and in the area below the fascia and rafters is the soffit.
While fascia isn’t a support for the roof it beautifies a house or other building, covering the ends of the roof’s rafters and its gables. It is considered part of the siding trim. Not ever roof has fascia because some designs rely on open-ended rafters. However, fascia helps to seal soffits, protecting them from pests and the effects of the weather.
Fascia is usually made of 1 inch thick lumber that has been treated to resist moisture and mold. It can be made of cedar or redwood but also of painted fir or pine. On older Victorian type houses the fascia on gables would often have curved ends or ornate designs at the peak.
Fascia seals the ends of the rafters and provides a firm place to attach gutters. Roofs usually extend at least an inch past the fascia and are covered at the end with a drip edge. As water rolls off the roof and into the gutters some might move back under the roof. The fascia keeps the water from the main body of the house.