The dumbbell row is an exercise that works all the major muscle groups in your back, including the atissimus dorsi, rhomboids, lower traps, and erector spinae. It will also work the shoulders and biceps as secondary muscles.
It is very difficult to train your back if you're training at home, the one arm dumbbell row is one of the few back exercises that only require a set of dumbbells and a low steady surface. But its not only a variation for home, it also has benefits for people who train at a fully equipped gym.
Being an unilateral exercise, an exercise that works only one side of the body each time, has several advantages. Although it takes twice the time to do the exercise, it allows you increased focus and concentration on the target muscle and help evening out strength imbalances between the sides of the body.
And because you're supported steadily by the bench, it allows you focus more on muscle contraction comparing to bent-over variations, since its easier to keep the torso stabilized.
The dumbbell row is very versatile, it allows you to work both widening and thickening of the back muscles. By pulling the dumbbell toward the hips you can hit more of the lower lats, while pulling in a straight line up will recruit more of the upper lats and rear delts.
Focus on your back muscles, you should move the upper arm with your lats, don't turn it into a curl. Think of your arm as merely a hook to connect the resistance to your lats and mid-back muscles.
Hold your back in place, don't rock it up and down and don't rotate it as you lift the weight. Rocking the back will shift the weight to your lower back, while rotating it will shift it to your hips. Both will make the exercise ineffective for your lats. If you unable to do this exercise without moving your back this is a sign you are using too much weight.
Do this exercise in a slow and controlled manner. Do not allow momentum to dictate the movement. This will make the exercise less effective and will make it harder to keep the back still throughout.
Try different neck positions. Experiment to find the position that works best for you, look a little forward or keep your head down in the same line as the spine. Find a position that is comfortable and doesn't put too much stress on the neck.