A combo simply involves a "combination" of two or more consecutive tricks performed during one wake crossing. The most common of these include a wake 180 back to air 360 back to back, a wake 360 back to back to air 360 back to back, or a wake backroll to air backroll.
More advanced tricks include a wake frontroll to air frontroll, a layout to front somersault, an invert front to invert back, a backroll to double(or more) air backroll, etc. Use your imagination and talent to come up with the wildest combo trick.