These days, there's a menu bar application for just about anything you can imagine -- checking Product Hunt, locking yourself out of all the distracting applications on your computer and even time tracking for work. When you need to quickly tweak settings or check the status of your Mac, the menu bar is usually the best place to do it.
By default, there are several menu bar applications preloaded on the Mac, most of them giving you quick access to the most commonly tweaked settings, such as volume, Wi-Fi and Bluetooth. Clicking on any of these will provide a list of quick access settings. For instance, clicking on the volume icon will provide you with a slider to fine-tune the volume level.
However, if you hold the option key while clicking some of the icons, it unveils an entirely different menu. Specifically, with volume settings, holding the option key will reveal a menu that will allow you to quickly change the input source and output device. For Wi-Fi and Bluetooth, the option key reveals more in-depth information on your device or devices you're connected to. The option key modifier also works with several third-party menu bar applications and will usually show settings that are still important, but probably not the stuff you regularly need to see or change.