wat is stereopsis?

Stereopsis is one of the processes of the human visual system that extracts depth information from a viewed scene and builds a three dimensional understanding of that scene. It makes use of the slight difference in perspective of one eye relative to the other.

It is worth noting that  there is away to get stereopsis out of the picture. Close one eye. Honest! Stereopsis is a two-eye process. Because it functions by exploiting the difference in perspective between two eyes, closing one shuts it down.