Parallax is a modern design effect used on images which dynamically adjusts the crop of an image as someone scrolls down a page, similar to how an image appears through the viewfinder of a camera.
Because it is a newer design feature, it is not yet supported by all web browsers. If a browser supports Parallax and you have this feature turned on, the image will appear with the Parallax effect; otherwise, the image will appear as a normal image.
Here is an example of what section images look like with Parallax turned off vs. on: