Often, just water. Chicken is sold by the pound. If you increase its weight by 5%, then you just got a close to free way of boosting profits by that amount.

They don't even hide the fact that they do it; most of them print it right on the packaging.

I quit buying supermarket chicken a long time ago because the quality is very poor. I trim the excess fat off chicken, and supermarket stuff has so much waste it was shocking.