He means mass. YoYou'll get bigger. Muscles will grow, you'll add fat (if your diet isnt good), and you'll retain a lot of water. When you come off them, you'll lose a lot of water, and be trying to keep whatever muscle you;ve gained (the hard bit)

If you dont drink enough water, you'll have liver (and possibly kidney) problems. You'll be dehydrated, wont gain anywhere near what ur expecting, and a shit load of other things. Try googling it. Theres loads of info out there.