Plain English Meaning
The pilot pulls back on the controls too hard or too soon just before touchdown, and instead of leveling off close to the runway the airplane floats back up a few feet. It is the opposite of touching down — the airplane briefly climbs when it should be settling.
Full Definition
A landing error in which the airplane climbs back upward during the round out (the transition from descent to the level attitude just above the runway) because the pilot raised the nose too quickly or too much, producing more lift than needed to arrest the descent. The airplane gains a small amount of altitude above the runway instead of settling smoothly toward it.