Astigmatism is defined as the eye creating two images instead of one image on the retina. This happens when the power of the eye isn't the same in all directions, often because the front of the eye (the cornea) is shaped more like the back of a spoon than a perfect sphere. Astigmatism does not affect the health of the eye, and eyes with astigmatism can be perfectly healthy. The result is that objects look blurry, but how much blur is different in different directions, which is why people with astigmatism often have trouble reading some letters but not others.
It can also cause headaches and eyestrain. In most cases it is easily fixed with glasses, contact lenses, or surgery.