The barbell rear delt row is a weight gaining exercise. It is targeting mostly your shoulder (deltoid) muscles. Also it has some slightly effect on your biceps, lats and your middle back.
For beginning pick a barbell with the weight that is comfortable for your weight and stand straight. The barbell should be with higher width then your shoulders. Whenever you are ready to start the exercise, bend slightly with your torso so that your torso and the floor should be almost parallel, with the bar in your arms. That will be your starting position, like on picture 1.
Start pulling the bar towards your chest as you breath in. While doing this, try to keep your upper arms closer to your torso. Make sure you are not using your biceps strength to pull up the barbell. The elbows are allowed to flare out from your body, just like on picture 2. That will be your ending position.
Once you are at your ending position, make a short pause then as you breath in slowly move the barbell to its starting position. Repeat this exercise in 4 series, while making 6-8 reps per each of the series. Make sure you do not overweight the barbell, because you can hurt your spine or some other muscles.