This exercise is used to build mass in both the biceps and the forearms.
Begin by standing with a dumbbell in each hand and your arms hanging to your side. Your palms should be facing inward towards your body. For ease of explanation this description will use the right arm to explain the exercise. Begin by taking your right arm and curling it across your body towards your left shoulder. Make sure not to twist your arm and keep your palm facing your body. Curl your arm up as far as you can and squeeze your bicep muscles at the top of the movement. Then slowly return the weight to the starting position and repeat with the right arm until your set is complete. Then repeat the movement with the left arm for the same amount of repetitions that you did with the right arm.
Do not swing the weight or use momentum to bring the weight to your shoulder. Perform this exercise slowly and really focus on feeling the bicep muscles contract and move the weight.