Here’s a specific example:
Suppose an airplane has a stall speed of 48 knots, and there’s a headwind of 60 knots. The plane doesn’t have to fly at 60+48=108 knots to keep from stalling. Since the plane is inside the air mass that’s moving at 60 knots, it only has to fly at 48 knots (indicated air speed) relative to the enclosing air mass. To an observer on the ground, though, the plane would appear to be flying at 48-60=-12 knots, i.e. it would appear to be moving backwards at 12 knots.