A yo-yo is an Earth toy consisting of a flattened spool wound with string that is spun down from and reeled up to the hand by motions of the wrist.
Tom Paris was responsible for making the yo-yo a temporary "ship-wide craze" aboard Voyager sometime prior to the crew's encounter with the Think Tank in 2375. At that time, Sheer Lunacy was the latest "crazy" – also thanks to Mr. Paris. (VOY: "Think Tank")