In a normal eye, light entering the eye is focussed by the cornea and the lens
... to form a sharp image at the back of the eye on the retina.
Astigmatism occurs when the cornea is slightly "rugby ball shaped"
The effect of this is that lines of different orientations ...
come to a focus at different points in the eye.
This results in blurred vision when looking in the distance and reading.
The degree and angle of astigmatism can vary from eye to eye.
If you have astigmatism you may notice that objects of certain orientations look clearer.