I've heard the word supertonic before. What does this mean?

Each of the seven scale degrees in the major and minor scales has been given a fancy name:TonicSupertonicMediantSubdominantDominantSubmediantLeading toneThese names are often used when referring to pitches in relation to the key that they're in, in order to describe their function. Their names can give clues: the leading tone wants to rise, or lead, to the tonic, such as the D in Eb major. The supertonic (super meaning 'above' as in superhero) wants to fall onto the tonic, like the D in C minor. The subdominant chord (sub meaning 'below' as in submarine) wants to rise to the dominant, as in the progression Dm - E in A minor, and so on.I hope this helps - try to start gradually using this terminology when discussing how notes relate to the key going forward!