MIC ATT is the switch to turn on or off the attenuation on the mic input. In simple terms, attenuation decreases the level of the input significantly. So in any situation where your audio input is too loud and distorting, you attenuate the signal and it should now record just fine. The most typical situation requiring attenuation is when you’d like to use a “line level” signal with a “mic in” input. Microphones generally have a very quiet signal, while line signals are quite robust & loud. Using the attenuator, the line signal from an audio mixer or the like can be reduced to where the mic input will be able to record it properly.
On my VX2100, the external audio input has a switch to select either a mic or a line signal. The main thing the switch does is to turn on the built-in attenuation for line inputs and turn it off for mic inputs. On some audio mixers, I’ve also seen the switch called a “pad.” But all three terms describe pretty much the same thing.
Hope this clears it up for you. Good luck with all your productions.