If you don't suffer from Astigmatism, your cornea and lens should be perfectly round in shape.
This allows light entering the eye to have one point of focus on the retina. This prevents the image from being blurry.
If you have Astigmatism, your cornea or lens will not be perfectly round. They may be slightly flattened, or rugby ball shaped.
Due to this, light entering the eye will be split into multiple points of focus on the retina. This often causes the image you see to appear blurry.