it's the position of the butterfly in the TB's. The bolts should be at the right hight to ensure that the butterflys are closed just the right amount (i.e. not so they jam). The way to make sure that are the right amount is to take the intake pipe off and do calibration for both TB's so that they are closing the right amount and synchronised.
this involves adjustment of both bolts on either side of the plenum and possibly the one in the middle and the throttle cable one as well, also a good time to clean the TB's.