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).