Parallax is an effect where the background content or image in this case, is moved at a different speed than the foreground content while scrolling. This creates an illusion of depth.

Hit the Preview on.. button and start scrolling the page to get a better idea of it.

To change the images please select them on the Elements pane. Since the .parallax-containers have a negative z-index to help create the effect you can't click on them... The best approach is to click on an element just above the image and the scroll down in the Elements Tree to select and edit them.