Cipher is right. Here's an illustration.
Moving your head back like that puts the low point of your swing (where the club bottoms out) too far behind the golf ball. With the irons, you'll be forever fighting chunks. With the driver you'll probably be topping a fair few.
There are a couple of simple things you can do to start getting the feel of keeping your head steady (... ier) through the swing. If you have a friend who will help, he can stand in front of you and just lightly hold the grip of a club against your left temple while you make some practice swings and then (gently) hit some balls, starting with half shots. Your focus is to keep the head against that shaft and not let it drift away. If you can't do that, he can press the grip more firmly against your right temple and actually "enforce" the head position to an extent. If you're practicing on your own, and are willing to look a bit strange, you can make a training aid from two driveway sticks and a pool noodle that will accomplish basically the same thing. It looks like this:
Hopefully how it works is obvious. You swing (starting slowly, again) while keeping your head on it until you have gone through the golf ball. Then you can come up.
