This refers to a built-in shortcut for a short-hopping air move, performed from a standing position by pressing the Attack+Jump buttons together. Hold a direction on the control stick if desired to pick specific airs (for example, hold down while tapping Attack+Jump for a short-hopping down air attack).
It used to be that short-hopping attacks had to be performed with a brief tap of the Jump button, then an Attack button press later, which is a lot harder. You might get a jump instead, pressing Jump too long; or get the wrong air move, juggling the control stick between steering your airborne fighter and choosing the air poke. With this command you always get a short-hopping air move. Short hop attacks do 85% of the damage done by an aerial attack during a full jump.
The only shortcoming is that the timing is immediate, so there will probably be midair end lag and no late coverage. It’s frequently better to manually short-hop and delay air moves to hit with the best timings, usually while descending, but this trick is great to remember when an instant short-hopping poke suffices, attacking while rising.