I have never heard this term. If I was to take a random shot:
The phase between the musical notes that can form chords and melodies. Some notes phase well together and envoke a feeling or emotion in us. Some notes played together would not agree with each other or sound "musically incorrect". aka phases don't "phase" each other. movement is awkard
In other words picture high school math class. The sine wave is what a musical frequency looks like. For it to go up, down, then back to where it started is completeing the phase cycle. the halfway point would be 180 degrees in phase. Squish the sine wave into twice the distance or stretch it twice as far, you now have the same note an octave higher or lower. ex. (A=220 hz, 440 hz, 880 hz etc). Music is math, the right distances between notes must be calculated.
Sine waves (notes) that are a certain distance apart together sound good when played together, therefore being in "harmonic phase".
That would be my take, I'm sure someone here can take it further