Bounce is a slang term used for the game action of returning a card from the field to its owner's hand.
Mostly, bouncing effects are found on Water attribute cards. Water likes to bounce both their enemies and allies to the point that it becomes an art form for them.
Bounce can be served as a cheap-but-temporary answer to remove a blocker or giving you another time to deal with a threat. On the other hand, bounce can be used to replay your resonators with ETB and [Enter] abilities, such as Izanagi, Keeper of the Seal and Nyarlathotep, the Usurper.