Single arm rows primarily work your Latissimus Dorsi, a muscle that covers the width of your middle and lower back.
To perform this exercise set yourself up with one hand on a box or bench. This should be high enough that with your other arm extended the weight will not reach the floor. You should have a flat back and a subtle bend in the knee.
From here drive your working arm up with a high elbow and keeping the arm close to the chest. Bring the arm back down and repeat.
You should ensure that you do not rotate at the bottom of the movement causing the weight to come closer to the floor. The correct bottom position is with your arm extended down, but with your back remaining flat.