An estrogen rebound is when you lower your estrogen level too far & then stop using the compound that lowered, your body will be in a big hurry to recover proper hormone balance & in doing so your estrogen levels will skyrocket temporarily in an effort to regain balance, when this happens you will experience some pretty extreme estrogen related side effects.
This can be softened or avoided with the use of Nolvadex, but again I say DO NOT USE AN AI OFF CYCLE.