Filtered most likely refers to the removal of certain harmonics that constitute a square wave. By filtering low order harmonics will round off the corners of a square wave.
Here is a diagram/graph of what is essentially a sine wave, referred to as the fundamental, with the 3rd, 5th, and 7th harmonics added, turning it into a square wave. By removing the harmonics from a square wave, it leaves behind the fundamental frequency that we know as a sine wave.
BTW, most electronics don't mind a modified sine wave. Reason is; the first thing the voltage hits is.... a power supply! Turning the AC into DC voltages. Mostly it's motors that run on AC. Very little electronics run on AC. Look for yourself; see what the firs thing is that the AC goes into. Usually a transformer and a DC power supply or several for that matter as most electronics utilize more than just one voltage. Just like your cell phone. The power supply changes the 120volt AC into 5 volt DC.
