The alternate dumbbell curl is exercise to build strength in your in your biceps muscles. This exercise makes your biceps constantly under tension, and personally this is the exercise i do first, when i have the biceps day, because it is great to warm up your muscles.
What you have to do is to pick same weight dumbbells in both of your hands, and do not make any other moves with your body like on picture 1. That will be your starting position.
Once you are ready to start with the exercise, first with your right hand start pushing the dumbbell upward to your shoulder level. While you are pushing the dumbbell, rotate the back hand like on picture 2. Once you have pushed the dumbbell to your shoulder level, make a short pause than start lowering back the dumbbell, while you are rotating the back of your hand again.
When you are done with your right hand, do the same work with your left hand. It is important to make equal reps per series for both of your hands, and make 4 series with 10-15 times of reps per series. For your first time, you should make 15 reps, with lower weight dumbbells, because in that case the blood can go to your muscles and will help you get used to more weight later.