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 crossup).
Faking a Jump-in
You don't have to do absolutely nothing when attempting an empty jump - you can do a slow attack that won't become active before landing, thus tricking the opponent into thinking you're doing a regular jump-in! Be sure to use attacks that have little to no landing recovery for this!