Parallax – is a visual difference in position of two objects relative to each other, which depends on the position and angle of the camera.
Take the pen in your hand and hold it in front of you, find some object that is at a distance from you (for example, a picture on the wall, a cabinet or a TV set). Now close the left eye, and “aim” the handle on this object. The handle partially covers it. Now open your right eye and close the left one. Now the handle does not cover the object or overlaps it in another place. This is the parallax effect. That’s why we need two eyes.)
The parallax effect helps a person to assess the depth in space. And it seriously complicates stitcher`s life, because it causes a lot of mistakes and distortions when splicing a panorama.