When you start typing a keybinding that has several parts to it (like
C-x C-s to save), if you pause after entering the first part (called the prefix key,
C-x in this example), then emacs shows the bit you have typed so far in the echo area (the line at the bottom of the screen where messages appear).
By default, emacs waits 1 second before echoing the keystroke, but I like to see it sooner. Of course this can be customised – add the following to your emacs config file to set it to 0.1 seconds:
;; Show unfinished keystrokes early. (setq echo-keystrokes 0.1)