What is a pentaprism? what job does it do in the camera?

Pentaprism is defined as a five-sided prism used chiefly in the viewfinders of single-lens reflex cameras. It has two silvered surfaces that gives a constant deviation of all rays of light through 90°.

Its purpose is to reverse the image laterally. When taking a picture, the camera lens reverses the vertically and laterally, the pentaprism re-inverts the image so that when you check on the image in the viewfinder, the image is correctly oriented.