Not sure what he meant exactly as I couldn't see what the lady was doing. You can hit groundstrokes (assuming she was hitting a groundstroke) with your feet planted as well as with a bit of lift... depending on the situation.
If you're serious about getting into it, I'd highly recommend finding a coach or hitting with someone who knows what they're doing. I've seen beginner/novice players play each other and they had the worst time ever. Spent most of the time picking up tennis balls and picking up bad habits.
I appreciate the insight. I think I found my " coach" below
He said that because she's completely off-balance. If she kept her feet on the ground, she could drive into the ball. By lifting her back foot up she's losing a ton of power on the shot. It could also cause the shoulders to fly open too soon and the shot might not go where you want it. But you're taught to step into the ball so it's defnitely a bad habit. It looks like she is compensating for more top spin by lifting the back foot to try to get under the ball. The back foot has to be planted and hers is going backwards. Watch Andre's back foot, it's always planted even if the front one is moving. His momentum is going forward as he squares up to the ball.
This is the same with golf weight transfer, balance, pronation, supination but the major difference is your not running like in tennis