The button in the Toolbox let us switch between "Quick Mask" and "Standard" editing modes.
Click
button to enter Quick Mask mode.
Edit mask using any painting, filling, and selecting tools.
Click
button to enter Standard Editing mode.
Tip. Just in case you didn't know, the Quick Masc mode is also available in Photoshop Elements that is way cheaper version. You just need a tiny workaround.