Complex sentence: The open cell foam is not only microporous, but its substance, melamine, is also extremely hard, which means that it is actually working like sandpaper, but on a very tiny scale, getting into tiny grooves and pits in the subject being cleaned.
Split into simpler sentences: The open cell foam is not only microporous, but its substance, melamine, is also extremely hard. This means it is actually working like sandpaper but on a very small scale, getting into tiny grooves and pits in the subject being cleaned.