This is a one legged Romanian Dead Lift, also known as a stiff legged deadlift (modified version). This exercise is used to work the glutes and is great if squats hurt your back. Also, it helps with imbalances because you’re on one leg, so each leg has to work.
*PLEASE don’t do any of these exercises unless you consult a doctor or trainer. I’m just giving you a taste of what I do. There is much more to come that is much more golf specific.