Explanation:

Since astigmatism is most likely to accompany by other refractive errors such as nearsightedness, farsightedness and presbyopia,  additional "cylinder" lens power to correct the difference between the powers of the two principal meridians of the eye.

Cylindrical lens has different angle of curvature in different planes (horizontal and vertical).

The cylinders refract the light rays at different angles from different meridians and focus them onto one spot on the retinal.

These glasses work well when the patient looks straight ahead but if you look to the side, things may still look distorted.