The bottom toolbar launches palettes and tools for adjusting your canvas.
The icons from left to right:
Layers palette: The 3 stacked squares icon will launch the Layers palette where you can select and reorder the layers in your design (such a graphics, text, photos, etc). See: All About the Layers Palette
Grid & Alignment palette: The grid icon will launch the Grid palette where you can adjust and turn on/off alignment grid features. (See: All About the Grid & Alignment Palette)
Before/After: The black/white square when clicked and held will show you what your design looked like when you started.
Undo/Redo arrows: Go back one or more steps. Or, put back one or more steps you undid.
Zoom Circles: Use the zoom + and - circles to increase or decrease the size of your canvas.
Percentage Drop-down: Click to quickly change to a specific %, or zoom to fit the screen.
Workspace: The monitor icon launches a Workspace palette that lets you customize the look of your canvas. (See: All About the Workspace Palette)