With frequency-modulated screening (non-periodic screening), the halftone dots are distributed pseudo-randomly. Different tonal values result from the varying number of dots of the same size on one surface.
FM screening produces better colour gradients and avoids Moiré effects.
Especially finer details can be displayed better with an FM screen than with an AM screen.
Moiré always occurs when screens overlap. Typical examples:
You can find further information here:
What is Moiré? And can I see Moiré in a proof?