An empty jump is a Jump-in except instead of attacking while airborne, land and do an attack instead.
An empty jump is useful for turning a jump-in into a mixup. Instead of always just blocking a jump-in standing, you change the timing to hit them with attacks that can't be blocked high (such as lows or throws)!
Depending on the attack done after landing, we can give more descriptive names to these empty jumps (eg. empty jump low, empty jump throw, empty jump overhead).
Faking a Jump-in
You don't have to do anything when attempting an empty jump - you can do a deep jump-in, but time the attack to just barely be too slow so the attack does not become active. You could also do an attack that will miss the opponent (such as jumping over the opponent and doing an attack that does not hit cross-up). Be sure to use attacks that have little to no landing recovery!