On my MacBook Pro, there are dedicated keys at the top of the keyboard for changing the brightness of the screen or volume of the audio. However there are only 16 discreet values, plus a 17th one for “off”. Sometimes you want a bit more granularity.
Hold down the option and the shift keys simultaneously, and then press the brightness or volume keys to adjust in ¼ increments instead (giving you 64 total settings).
(Also, if you hold down just the option key while pressing brightness or volume, it will bring up the appropriate system preference panel.)