Stacking is a useful and common feature in most of advanced photo catalog/archiving software and consists in grouping multiple photos showing just one of them (the one that is put on top of the stack).
It is useful when you have multiple shots of the same subject and want to compact the display.
Each software have its own way of handling stacking. This article describes in detail how Adobe Lightroom handles stacks, that is not so intuitive (you'll see that depending on the function, the stack can be treated in a way or in another).
Creating the stack
Just select the images that you want to stack and via the menu or with right-click stack them. There is also a shortcut (Ctrl-G for Windows).
There is also an Auto Stack feature (Menu "Photo -> Stacking -> Auto Stack by Capture time") that creates stacks with photos shot within a certain time interval. Useful for example when you shot with bracketing (exposure or white balance bracketing where camera takes 3/5 shots with different settings) or when you make a burst shot.
Viewing the Stack
The stack can be collapsed ("closed") where just the top image is shown together with a number reporting the number of images the stack is made of:
or expanded where all images are shown:
You can collapse/expand in several ways: with menu command ("Photo -> Stacking"), right-clicking on the stack (Stacking), with a shortcut (S) or by clicking on the number of photos of the stack (top left corner of the top photo in the stack).
You can change the photo that appears as the top one via menu, right-click or shortcuts (Shift-[ and Shift-]).