Earlier quoted context omitted.
A cluster of Emacs keybindings that share a common prefix, implemented using the hydra library: https://github.com/abo-abo/hydra
So it's sort of like a mode switch? You give the prefix, and then you can give it a whole series of commands, based on a crib sheet that it shows you, that have that common prefix - without having to retype the prefix - before passing a different key to drop out of the mode again? That does sound potentially handy, if I have it right.
Also, it's possible to configure it so that there's a delay between the time you type a keystroke and the menu appears, so that if you remember the key-sequence you need to type to perform a certain action and you type it fast enough, then the menu won't pop-up, but if you forget the full sequence then you can type just the first keystroke and then wait a bit and the menu will pop up and remind you of what needs to be typed to execute the action you want.
It's super handy, and is one of my all-time-favorite emacs packages.