i think in terms of drive pedals, mids typically refer to frequency from 500Hz to 2000Hz. midhumps can be achieved 2 ways, one being boosting these frequency above the rest, and the other being cutting the rest of the frequencies apart from the mid.
mid boosts are really obvious in some pedals, like the ibanez tubescreamer. it gives the pedal a very round and fat tone but not too muddy (cause apart from the trebles, some bass is removed too).
the human ear is most receptive to frequencies around 1000Hz (meaning if many frequencies are played simultaneously at the same actual volume on paper, the frequencies around 1000Hz would be perceived to be louder than the rest to us)
apart from making the tone of the guitar sweeter (objective), pedals that have the characteristics of boosting these mids are generally used to make the lead guitar cut through the mix during solos and solo riffs.
its not a good idea to use mid boosted pedals to play backing rhythm as the singer sings because it fights for "sound space" and distracts listeners from the vocals. thus people scoop mids for guitar for rhythm when the vocalist is singing.