What is a Low Pass Filter

I can not fin a definition that a newbie like me can understand searching Audiogon. My Subwoofer, Sonus Faber Cremona, has 2 dials in the back. One for Low Pass Filter, switchable from 38-65Hz 6 positions and one for Gain that reads min to max.

More Info:
http://www.sumikoaudio.net/sonus/prod_cremonasubwoofer.htm

Thanks in advance for any help,
David