Brake fade is when the brakes overheat usually from hard driving, it can feel like your brakes do nothing. this can also happen, although rare, in the heat of summer after a couple of stops. It is a normal occourence, unless it happens often, then you may have faulty brake pads or if your brake rotors were installed on the wrong side (very common) between left and right, then the venting may not be rotating in the right direction and not cooling efficiently enough.
also possible that your combination valve may be dirty, (includes the proportioning valve and metering valve for drum brakes all in one)
also possible sticking in the ABS system (if you have one)