The shelf is a space at the located at the bottom of the screen on a Google Chromebook that contains various icons. It allows users to quickly access their favorite apps and websites, similar to the Windows Quick Launch feature or items pinned to the Windows Taskbar. The image below shows an example of the Chromebook shelf.
To launch an item pinned the Chromebook's shelf using shortcut keys, hold the Alt key and press the number that corresponds to the item's position. For instance, in the image below, Alt+1 would launch Chrome, Alt+3 would launch the calculator, etc.