Super tonic just means above the tonic--the supertonic is part of the dominant harmony(as opposed to the submediant, which is the relative minor and part of the Tonic harmony)--that is to say, it is related to the G7--in fact, play the two chords together and you'll have a G9--in the key of C--

If you know about this, you'll understand why one set of chords to a particular song has, say, a D minor leading to the G7, while another just has the G7--Also, if you want to play a song in G, and you find the chords in Bb and see "Cm7-5 F11" you won't get all freaky, you'll know that it is just the old SuperTonic to Dominant and you'll play Am C7--