What is "optical refractive index" with regard to seeing/atmospheric distortion?

So I'm doing my physics project on adaptive optics, which is a technology used for ground based telescopes, and wikipedia has defined seeing as "the blurring and twinkling of astronomical objects such as stars, caused by turbulent air in the atmosphere varying the optical refractive index." I clicked on the link for optical refractive index, and I don't know how varying the optical refractive index would cause blurring; is it that there wouldn't be a single focal point? I still don't get it, so any help would be greatly appreciated! Thanks :)