That’s what’s known as a “bass rolloff” or “low end rolloff” switch. Basically it will reduce the low end rumble you get from air conditioners and such. It can, however, make your audio sound “thin” and you’ll need to EQ in post. I rarely use rolloff anymore with the great audio plugins for removing noise. If you’re going to use noise removal software, be sure to record at least 10 seconds of “room tone” to get your sample from.
Your best bet is to choose the proper location with as little noise as possible. Then turn off such items as room air conditioners/blowers (call facilities people at offices for help with that) unplug refrigerators (put your keys in the fridge so you remember to plug it back in).
And don’t forget really good headphones so you can really hear what the mic is picking up.
And practice with it before the actual shoot! Try some recordings at home with and without the various settings to see what you get.
Steve

