So within System Preferences> Accessibility, click on the “Keyboard” tab from the left-hand list, and you’ll see a checkbox labeled “Enable Sticky Keys.”
This lets the user input keyboard shortcuts as a sequence instead of having to press the keys all at once. So if you’re looking to quit an application, for example, you can press Command, release that key, and then press Q. As you hit any of the modifier keys, their corresponding symbols will be shown on your screen until you complete the shortcut.
The symbols above, just so you know, are for the keys that can be invoked through this feature—Function, Shift, Control, Option, and Command (from left to right).
Additionally, if you select “Options” within that System Preferences pane (shown in yellow on my first screenshot), you’ll get some controls for how Sticky Keys looks and functions.
I personally like turning on the “Press the Shift key five times to toggle Sticky Keys” option, as that gives users some power over when they’d like the extra assistance. In any case, I’ve found this feature to be incredibly handy for anyone having trouble with holding down those pesky modifier keys. Everyone should be able to be a power user!