The toolbar contains some of the most common functions you'll use in FretPet. In the first section are controls to modify chords. Next is playback control, where you can start, stop, loop, solo, and more. Moving on, you'll see a few handy display mode buttons. And finally, the "+" button is used to add the current chord to your chord progression.
After you get the hang of the Toolbar you should check out the Filters menu. That's where the really interesting spells are hidden.