A traditional MacBook keyboard used to have 3 buttons to adjust sound volume: Mute, Volume Up and Volume Down. These buttons were all mapped to the function keys of the keyboard.
Since 2016, the MacBook Pro started shipping with a component called the Touch Bar (see image below). The Touch Bar replaces to top row of keys in the keyboard (the escape key, the function keys and the power button) with a touch sensitive OLED strip, including a finger print sensor which also acts as the power button. In the Touch Bar, the traditional function keys moved in to what is called the Control Strip, a small region in the far right of the Touch Bar which is collapsed by default but can be expanded by hitting an arrow button.
The volume controls are also located in that region, but when collapsed only 2 buttons are shown: Mute and Volume Slider. To make quick volume adjustments, you can tap the Volume Slider button and use the slider to adjust volume (you can do this in one operation). This works pretty fast, but I find it hard to quickly adjust the volume step-by-step.
Now, I accidentally discovered that there seems to be an alternative interaction which allows for very quick and accurate volume adjustments: swiping the Volume Slider button (see video embedded below). Really nice!
BTW, this interaction also applies to adjusting screen brightness.