Waveform phasing is a phenomenon that occurs when multiple oscillators are present in a signal. Even once multiple oscillators are in tune with one another, they will not be identical, their waveforms will phase against each other ever so slightly. In a drone, this is heard as phasing, in a more rhythmic setting this can be perceived as changes in volume and/or presence throughout a musical part.
A useful analogy to consider is two strings, on a guitar for example, tuned to the same pitch. When plucked together their sound will not be uniform or singular.
To avoid waveform phasing, you can turn sync on while leaving the frequency knob in the center position.
Here are some examples of waveform phasing.