The RDL is used to strengthen the hamstrings and low back.
Unlike the standard deadlift the RDL starts at the hip. From here you bend at the knees slightly with your torso vertical. Keeping your knees bent tighten your back and keep it arched, then lower the bar down your thighs as far as you can whilst still maintaining the arch in the lower back, pushing your hipsback. Then return the bar back to the start position. Remember to keep the bar as close to you as possible.
The RDL strengthens the hamstrings, glutes and low back and teaches the correct mechanics of a hip hinge. Upper back will also be exposed to a lot of work due to the focus on keeping the bar close to the body.
Reps between 3-8 are most commonly used but higher reps can be used with lighter weights if your goal is reinforcing correct mechanics of the lift. Load should not exceed that of which the arch in the lower back can be maintained.