What is astigmatism?

It is the most common of the three refractive errors (myopia, hyperopia and astigmatism) affecting eyes.  Literally, the word means "no point focus" which means that light coming in is distorted by the amount of the astigmatism.

It is theoretically present in all eyes, because a truly non-astigmatic eye would have to have perfectly spherical refractive components (cornea and crystalline lens), but for to be clinically relevant, or prescribable, the asphericity must be at least 0.25 diopters.

You can tell how much astigmatism you have by looking at the second number in your spectacle prescription.