Complex sentence: If the relative difference of the input and output waveforms is small, then the converter can produce "subharmonics" which is noise that occurs at a frequency below the output frequency, which cannot be filtered by load inductance.
Split into simpler sentences: If the relative difference between the input and output waveforms is small, then the converter can produce "subharmonics". Subharmonic noise occurs at a frequency below the output frequency, and cannot be filtered by load inductance.