The bottom toolbar launches palettes and tools for adjusting your canvas.
The icons from left to right:
Settings wheel icon: This icon will launch two settings controls:
Alignment palette where you can adjust and turn on/off alignment grid features. (See: All About the Grid & Alignment Palette)
Workspace palette: Here you can customize the look of your canvas. (See: All About the Workspace Palette)
Before/After: The black/white square when clicked and held will show you what your design looked like when you started.
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.
Undo/Redo arrows: Go back one or more steps. Or, put back one or more steps you undid.
Pages icon: Add additional pages to your project up to a maximum of 30.
Layers panel: The 3 stacked squares icon will launch the Layers panel where you can select and reorder the layers in your design (such a graphics, text, photos, etc).