It means that if person tries to Focus on 1 small point, But person can't catch that small point. It will let him see whole part. May be if you can understand Physics so I can explain you in another way. that Our Eyes is like lens : If there are many parallel rays are coming to crystalline lens, then all the rays will got focus on 1 point. This is for Normal Eyes. so Person can see thing very perfect(regular).
But if person has Astigmatism so, all the rays go Crystalline(won't got focus on one point (Won't centralized) ), then person can not see clear.
An astigmatism usually refers to the eyes. It can be that your eye/eyes are slightly misshapen, maybe oval instead of round, and this causes visual problems. It is not near sighted or far sighted, but more of a defect in the shape of the eye that causes visual problems.