The Button bar is your main access to change measurement settings, to configure the current active view and to adapt the display settings to your needs. The Button bar is context sensitive. Thus, the available buttons depend on the active view.
The title of the current menu (Main in this example) is displayed on top of the Button bar.
When you have opened a submenu, tap the title of that menu until you get back to the main menu.
The buttons with a white triangle in the lower right corner show a submenu when tapped. Some buttons even may have several submenus.
To close the submenus, tap the header of the Button bar or press the Esc key until Main is displayed.