Astigmatism is a refractive error of the eye, also known as cylinder. It occurs due to deviation from curvature of the retina or eye lens. It usually appears accompanied by myopia or presbyopia. Those suffering from astigmatism have difficulties with the absence of light (at night).
If you do not have astigmatism, the lines on the image above will all be of the same width, equally distanced from one another. On the other hand, if you see something like two triangles with its top in the middle of the drawing which rotate as you move your head, you probably have astigmatism.
As with the previous image, those with healthy eyes will see the lines on this image equally thick and separated. If you suffer from astigmatism, some of the lines will appear thicker and darker.