How to Stack: Stacking is a method of organization which helps you to bundle "like" images together. That being images which are extremely similar in content, part of a multiple shot series (example: HDR series, time lapse series or rapid fire series). The idea being if you have dozens of images that are nearly the same or need to be together to make sense, then why fill up your entire screen with them all. A single representative image will tell you all you need to know about the remaining "x" number of images.
To stack images you simply select all the images in a series or group which you wish to add. You do this by clicking on an image (hold ctrl pc /alt mac) to continue selecting multiple images. You can also select a large group of images in order by clicking the first images (hold shift) and then bookend the entire series by clicking again on the last image.
Once your images are highlighted, you can right click > stacking > group into stack. You'll now see all your images compress into a single image with an "x of y" box in the corner showing you the total number of images in the stack.
Stacking Automatically: So what if you've shot hundreds of photographs and every photograph is part of a series?