Its the effective angle the wind is hitting you, if the wind is directly in your face or at your back its 0. At other angles, it gets more complicated because it is also relative to your speed, as in if I'm standing still with a 90 degree wind blowing at me, the angle of that wind will approach 0 as my speed increases. Its one of the reason they recommend deep dish or tri spoke wheels more to faster riders, less of an effective yaw angle.