You're right, it's not plain English. I remember scratching my head on this one.
I'm enclosing a GSP sketch that will illustrate it. If you don't have GSP, write me back.
If AC means the distance between points A and C, BD means the distance between B and D and so on, and if a distance in the direction of the ray AB is positive and a distance in the direction of the ray BA is negative, then a harmonic set is one where