`which-key' is a minor mode for Emacs that displays the key bindings following your currently entered incomplete command (a prefix) in a popup. For example, after enabling the minor mode if you enter C-x and wait for the default of 1 second the minibuffer will expand with all of the available key bindings that follow C-x (or as many as space allows given your settings). This includes prefixes like C-x 8 which are shown in a different face.