What does the 10dB switch on a mic do?
The only stupid question is one that's not asked!
The -10 dB switch you see on some microphones is a "pad" switch, which attenuates the output of the microphone by 10dB. You would use that when recording a very loud source where the microphone output is overdriving your microphone preamp even with the preamp set to the minimum levels. Some microphone preamps have pads on them as well. For really loud drums, you may find you need the pad engaged on both the microphone AND the preamp at the same time to keep from clipping your A/D converters.
For microphones that don't have a built-in pad, you can always buy external pads that have XLR connectors on both ends and connect between the microphone and the microphone pre-amp. I have several of these that are often necessary when recording drums and using microphones that don't have their own built-in pad which feed preamps without a pad (or sometimes even with the preamp pad, I still need an extra pad for the really heavy hitting drummers).