Naw, it sounds more complicated than it is. First thing is understanding major scale theory. That is pretty simple. C major scale consists of C(I), D(ii), E(iii), F(IV), G(V), A(vi), B(vii) with the numbers denoting the degrees of the scale. So D is the 2nd of C, E is the 3rd of C, etc. Think Do rei mi, etc. That's the major scale and its various notes or degrees. When we stack the notes of the major scale in thirds we get the following chords Cmaj, Dmin, Emin, Fmaj, Gmaj, Amin, Bmin. Those are just the basic triads, root, 3rd & 5th. You are familiar with these chords. In jazz, we add another note to get 7th chords (root, 3rd, 5th & 7th), so Cmaj7, Dmin7, Emin7, Fmaj7, G7, Amin7, Bmin7b5 are the chords in the C major family. As long as we stick with those chords we stay in the key of C. Most pop/rock/country/blues tunes stick to just one key. For jazz, the most important notes in a chord are the 3rd and 7th. The bass player takes care of roots so guitarists typically don't have to worry about them. We focus on 3rds and 7ths the most when playing chords or lines, often.