Youll be using the tools on this panel more than any others when modeling; these tools control your shapes appearance by applying modifiers to its polygons; anything from meshsmooths (smoothing the surface through iterations of polygons) to extrusions (drawing out one or more faces) to bends and tapers (literally bending or squeezing your shapes) and much, much more. Theres a default set of eight of the most commonly-used buttons, but you can customize it to display whichever tools you prefer.
The easiest way to get to the majority of the modifiers, however, is through the dropdown menu listing every modifier available. Once youve selected a modifier, the window below will display the shape/object youve selected and a hierarchy of the modifiers applied to it. Below that, the expandable editing panels let you change the settings of how they affect your shapes.