The default toolbar offers a wide range of useful tools and it can be customized with a number of additional items. You can also rearrange the order in which they are displayed.
Play/Pause. Resume Live View if paused. Adjust focus while viewing the focus window. When the focus has been suitably adjusted, press the pause button. Note that a Play/Pause option is also available on-screen in the Live View window.
Turn Overlay On. Toggle an Overlay on or off independently of the Overlay tool's Show option.
Alignment Function. Toggle Grids and Guidelines on or off (depending on preferences). Guidelines can be adjusted on-screen by clicking and dragging.
RGB. Toggle between a color or mono Live View image stream. Switch to mono when color is distracting during composition or when color fringing, such as longitudinal chromatic aberration, is interfering with focusing. Note that a mono image may appear grainier compared to a color image, however, this does affect the captured image.
DOF/EPV. Select to stop down the lens to the taking aperture and disable auto-gained Live View stream. Note that behavior is highly dependant on camera model and exposure mode in use.
Preferences. Shortcut to preferences. Live View duration, certain camera model configuration settings, grids, and guides can be altered from the Capture and Crop tabs respectively.
NOTE: Click on the Hammer and Wrench icon or click anywhere in the toolbar area and press Ctrl/right-click and select Customize Toolbar....