What does it do and how do you use it?

it allows you to apply effects to a pad for short stabs rather than continuously

you do it by holding Tap Tempo, and then simultaneously hitting the pad of the effect you wish to apply

for example, if i had Reverb assigned to Pad A3 (Snare), i could let the drum pattern play and when i felt like giving the snare some booming reverb, i'd just hit both Tap Tempo and MFX buttons, then take my finger off MFX and tap MFX again when i want to affect the snare