The parallax effect is a visual effect when viewing 3D objects from a 2D screen.
When the viewer moves parallel to the 2D screen (eg: viewing scenery from a window on a moving car), the far objects’ position will move slower than the near objects. As a result, the relative positions of objects on the 2D screen changes.
The parallax effect can be mimicked on a website’s background images/videos. When viewer scrolls, the background images/videos move slower compared to the images’ container. This creates an illusion that viewers are viewing through a window with images/videos at a distance.