The Toolbox is fixed to the top-left of the Layout Panel and contains the tools that operate on the workspace. The selected tool determines what SWiSH does when you click and drag the mouse on the workspace. For example, when the 'Line' tool is selected, a click and drag will create a line object, but when the 'Rectangle' tool is selected, a rectangular object will be created.
Double-clicking on a Toolbox tool locks them in until another tool is selected.
For more information see the Tools section.