The `which-key' mode 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.