This is probably one of the top 3 exercises people neglect (and number 1 for the shoulders!). In fact I was the same way.
The shoulder (specifically the deltoid muscle) has 3 components (front, side, and rear). Most people workout the front and the side delts (short for deltoid), but not as much the rear.
Not working the rear delts will lead to an imbalanced shoulder shoulder muscle so next time you are doing shoulders, throw this into your routine!
Get a couple light dumbbells and hold them at your sides. Bend over while keeping your natural curve in your lower back. You can bend your knees a touch too. Then bring the dumbbells up and out to the sides while keeping your elbows mostly straight and get a good squeeze at the top.