When you play major intervals, chords or keys (versus minor) - they have a whole tone step to the 3rd and 6th. Some examples:
- C major 3rd = C + E
- D major 3rd = D + F#
- A major 6th = A + F#
- E flat major 6th = Eb + C
- G major chord = G + B + D
- B flat major chord = Bb + D + F
Read more: Wikipedia