The purpose of this neck exercise is to help bring your head back to a more upright postural position. It'll really stretch and exercise the base of the neck. As you get older, this will help prevent that forward slumping of posture and that sterotypical "old age" look.
Start with your hands behind your neck. Do not interlock your fingers, so that you can extend your head all the way back. Keep your elbows together, this will prevent strain to the shoulders.
Now, stretch all the way back. Use your arms to give your head and neck resistance. You should feel a stretch under your arms and at the back of your neck