Not Medical Advice: Astigmatism is probably the most misunderstood vision problem. For starters, it's called "astigmatism," not "stigmatism." (You don't have "a stigmatism" — you have astigmatism.)
Stigmatism has to do with lenses and the refraction of light. It is the image-formation property of an optical system which focuses a single point source in object space into a single point in image space.
Astigmatism is a refractive error, meaning it is not an eye disease or eye health problem; it's simply a problem with how the eye focuses light.
As per American Academy of Opthalmology, astigmatism is an imperfection in the curvature of your cornea — the clear, round dome covering the eye's iris and pupil — or in the shape of the eye's lens. Normally, the cornea and lens are smooth and curved equally in all directions, helping to focus light rays sharply onto the retina at the back of your eye. However, if your cornea or lens isn't smooth and evenly curved, light rays aren't refracted properly.
Slight amounts of astigmatism usually don't affect vision and don't require treatment. However, some patients with astigmatism seem to have significant difficulty driving at night or working under certain light conditions. Check out some tips for minimizing unpleasant symptoms at About.com.