The Reverse Fly will target your rear shoulders (posterior delts), as well as your back muscles (rhomboids, trapezius and latissimus dorsi.) This can be very beneficial to posture and to prevent back pain.
To set up for the reverse fly place your feet approximately hip width apart and hold two dumbbells with your arms extended downwards with a slight bend in the elbow. Stand with a slight bend in the knee, hinge at the hips to bring your chest over and your bum backwards. Ensure that your back remains flat throughout. From here, drive your arms out wide squeezing the muscles in between the shoulder blades, before returning back down.
In order to progress this movement you can adjust the angle of your arms such that your pinky fingers are higher than your thumbs. Keep the weight light so that you can really nail your form and prevent having to use momentum to lift the weights into the air.