Fascia is the board that runs along the edge of the roof where it meets the walls. It supports the gutters and closes off the roofline, and because it sits out in full weather, it needs thorough prep and a quality coat of paint to keep protecting the structure behind it.
Rotted or peeling fascia is often the first sign that water has found its way in. Catching it early means a repaint instead of a rebuild, which is another reason exterior inspections before a quote pay for themselves.