An aggressive, posturing jump (or hop) with no attack performed. This is most useful to score grabs, since grounded targets who raise shields anticipating jump-in air moves are vulnerable to attackers who fast-fall and grab.
Empty-jumping/hopping can also be used as bait against reactive challengers. If someone is twitchy against jump-ins—always ready with an up special, up tilt, up smash, just generally up—you can hop aggressively without attacking, air-dodging just before you get to them (neutral, for lower landing lag). If you’ve read them correctly, their reactive anti-air will whiff through the air dodge’s intangible period, and you’ll land in time to grab or strike them!