Tonic is the first scale step--
Supertonic is the second
Mediant is the third
SubDominant is the fourth
Dominant is the fifth
Submediant is the sixth
Subtonic is the seventh

In the Key of C--the Chords are:

Tonic:             C major
SuperTonic:    D minor
Mediant;         Eminor
SubDominant: F major
Dominant:       G or G7
SuperTonic:    A minor
SubTonic:       B diminished


Three chord major key songs, are often C-F-G7--which is Tonic Subdominant and Dominant--

Each of these chords has it's own special function in the key, and the names refer to that--If you've played music for a while, you know about this, even though you may not know these names--

It helps to learn the names though--you pretty much need to know them if you are working with chord substitutions, and, at least with guitar, it helps to know the relationship between the chords in a tune when you are trying to figure out the best place to play it on the neck (I call this "enharmonic transposition")--

As for you, GUEST--if you think what you know would get you by with Merle or Willie, well--both of them could play about anything that came down the pike, and in any key--

Another thing about guitar--there is some stuff that can't be transposed --The Jimmy Reed thing, for instance--