A shortcut key is a key combination by means of which a tool can be activated (for example, SHIFT+CTRL+DEL for Delete). You can even use the shortcut key to activate a tool when the icon of the tool is not on the Actionbar or toolbar or when the toolbar is hidden. Some shortcut keys are already defined by default. You can also define your own shortcut keys. Shortcut keys are displayed in the ToolTips that appear when you position the cursor over an icon.
You can see a list with all predefined shortcuts by clicking - Shortcut Keys on the title bar on the right.