A bounceback is usually a discount vacation offer that Disney offers you on a future trip while you are on your current trip. It *has* to be booked during your current trip.
For example lets say you were are Disney right now. In your room you may find a flyer that says if you book for a future trip period (usually specific dates) you will receive a certain discount (perhaps 40% off deluxe, etc.) or a specific promotion (i.e.
free dining). But you have to book the said bounceback offer before you check out of your resort. There isn't always a bounceback offer available.