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
Settings wheel: This icon will launch two settings controls:
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.