Astigmatism is a result of the lens in your eye not being perfectly spherical ie, slightly cylindrical; this results in the image on your retina being in focus in one orientation and (slightly) out of focus in other orientations. In good light it may not be noticable but in poor light it can start to be more obvious and can result, say, in text being difficult to read. The solution is to make the optician's lens cylindrical at right-angles to your eye-lens - the axis is the angle your eye-lens is cylindrical and tells the lens maker at what angle to make the correction lens cylindrical.