The hotbox contains every action available in the Maya interface. It appears when you hold down the space bar.
The hotbox has three main functions:
To select an action from the hotbox
 above,
            below, to the left, to the right, or in the center of the hotbox
            to show marking menus.
  above,
            below, to the left, to the right, or in the center of the hotbox
            to show marking menus.