Major 6 chords use the scale degrees: 1, 3, 5, 6
D6 uses the notes: D, F# A, B
Major 6 chords can be played in place of almost all major chords and as chords I, IV and V in major keys.
Major 6th chords can also be considered as relative minor 7th chords, D6 and Bm7 have the same notes in so you can play this inversion in place of either of these two chords.
Tune in tomorrow for another Guitar Chord Of The Day