Trainability is a measure of how well you can adapt to certain stimuli. So with aerobic trainability, we are looking at how quickly your VO2max (your ability to carry oxygen) will improve in response to aerobic training.
If you score medium, you have average trainability. If you come back as high/very-high, you are a high responder to aerobic training; conversely if you come back as low/very-low, you don't respond as well to this type of training.
It isn't possible to predict a specific VO2max score for an individual (e.g. 70ml/kg/min) based on the DNAFit test.