I would assume that she picks them up with the lightest touch rather then an "ask tell demand" situation. I would think it means she listens to every movement of your seat which would actually make it a little bit harder for someone to ride....(right off the bat)
To me when a person says light on cues I would expect a horse who will move off the lightest of pressure. I have several horses who are so very very light and will move off my leg at any speed. I barely need to touch them and they do it NOW not is a step or stride but now.