A harmonic is a pitch that is an integer multiple of the fundamental frequency. In other terms: on an acoustic string instrument, when you play a note, say 110 Hz, the string would resonate mostly at 110 Hz, but it would also have simultaneous resonances at approximately 220, 330, 440Hz and so on, that get progressively weaker. Those simultaneous resonances are harmonics. Here is a graph that somewhat explains it, the x axis is Hz, the y axis amplitude (volume) Now on your softsynth, there are three things that knob could do:
- Amplify all harmonics, or some of them. This would make the sound brighter, to put it simply.
- Change which harmonics are accentuated. This would change the character of the sound, hard to be any more specific.
- Change the pitches of the harmonics. This is unlikely. It would certainly allow for some weird sounds.
I suggest playing with your knob, see what happens to the sound. If necessary, use a spectrometer to figure it out. Or if there is a manual of the thing, maybe that could help.