The stereo baseline is the distance between the two points at which the left- and right image that form a stereo pair are taken. In a stereo camera it’s the distance between the two lenses. Traditionally, the distance corresponds to the distance between the eyes of a human being, which is on average 6,5 cm. This results in a natural depth effect when viewing a stereo image.
A stereo image with a larger baseline is called a hyper stereo. A larger baseline is needed when photographing large, distant objects such as mountains or large buildings. In this case the normal baseline of 6,5 cm will result in flat images that lack depth.