Maybe you are aware of dominant and tonic, the two solid navigational waypoints in the common scales. Above the tonic there’s a note roughly midway up to the dominant. This is the “hub”. We might learn this as a third waypoint. It is somewhat less solid, because it varies slightly (from one melody to another) to give the major-minor feel to the music.
The interval the hub makes with the tonic is a third. That interval is either four “frets” (semitones) or three frets. The four-fret third is a major third (M3), and the three-fret third is minor third (m3).