The fascia is the long, flat board nailed across the ends of the rafter or truss tails along the eave. It closes off the ends of the framing, gives the roof edge a straight finished line, and is what the gutters hang from. On a gable roof the matching board running up the sloped edge is the rake board.
Many builders install it in two layers. The sub-fascia is a structural 2x — usually a 2x6 or 2x8 — nailed to plumb-cut rafter tails to straighten them and give gutter brackets something solid to grab. The finish fascia goes over it: primed 1x6 or 1x8 pine, cedar, PVC, fiber cement, or aluminum coil stock bent to wrap the wood. The fascia should be wide enough to cover the tail cut and the edge of the roof sheathing, and it is set so the soffit can tuck into a groove or behind its bottom edge.
At the top, the roofing drip edge laps over the fascia so water drips into the gutter instead of running behind it. Leaking gutters and missing drip edge are the main causes of fascia rot; repaint or replace wood fascia at the first sign of soft spots, because the rafter tails behind it are next.