An empty jump is a Jump-in except instead of attacking while airborne, land and do an attack instead.
Application
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 mixups (eg. empty jump low, empty jump throw, empty jump crossup).