As Rob said, it’s basicall

As Rob said, it’s basically cutting down on the bass fequencies. It’s not cutting them completely out though. That would sound very unnatural. The reason for this is to reduce things that you mentioned like rumble, handling, pop and wind noise which are mainly found in the low frequencies.

Unless you are recording outside in a pretty strong wind, you should probably leave the roll-off switch set to “flat”.

I have used the ME66 and find that the roll-off does almost nothing for handling noise since the chassis of this mic is so thin. Because of that, the resonance of handling noise is a much higher frequency than most mics and isn’t reduced by the roll-off. BTW, I wouldn’t suggest using that mic as a handheld mic at all because of the amount of handling noise it has. It works fine on a handheld boom or camera as long as it has a good shockmount or a stationary boom without a shock-mount but don’t even try to hold it.

Jeff