An Instant Air Dash (IAD) is quickly jumping and air dashing from a standing position.
Performing an IAD
Traditionally it is performed by entering 
.
An air dash is performed by entering while airborne. The counts as both a jump input and a forward input. You then enter to finish the air dash input.
Minimum Height
If you have problems performing this, remember that you can not immediately air dash after jumping, you must reach a certain height before you can air dash. If you have problems doing it, try delaying slightly.
Mid-Combo IAD
Dash Macros
Games with button macros for dashes can do the same thing by pressing any upward direction, then pressing the dash macro.