First of all, the thin shot is not always a bad shot. Some of the better golfers even play it intensionally at times, but for most golfers it is a mis-hit, and so avoiding the thin shot in golf can be a priority for some, especially if it is a frequently recurring problem. In any case, at least one swing fault will be a contributory factor, it often goes hand-in-hand with the dreaded golf slice.
So what exactly is a thin shot in golf, apart from being the opposite of a fat shot?
The simplest explanation is when the bottom of the club head, or blade, makes contact too high with the golf ball, usually at the midway point or slightly below. This sends the ball off on a low trajectory which usually ends up traveling further than intended and the direction of the shot is often unpredictable leading to an inconsistent golf swing.
Thin shots are generally caused by players pulling their body up prior to impact, common among golfers that hunch their shoulders forward at address, or releasing the club to early so that the club head gets ahead of the hands before impact.
Having the wrong swing plane can also cause thin shots in golf. Standing too far from the ball can make the golfer hunch their shoulders forward making them have to stretch in order to reach the ball, this produce a flatter swing plane regardless of the golf club used. Standing too upright produces a lot of fault symptoms, normally a slice because the club head will follow an out-to-in swing plane, but a thin shot is also a common result.
Swaying towards your back foot during the back swing and not swaying back during the downswing enough can also cause a thin shot. Just try and avoid swaying all together, it's not necessary and too hard to judge right that it's best not to do it at all. Swaying is not the same thing as the weight transfer towards your back foot on the back swing and towards your front foot on the downswing, this is different from swaying. You can easily improve your golf swing just by eliminating the unwanted swaying motion.
Your shoulders, point too far left or right, at address can also produce thin shots and is often overlooked as a source of this mis-hit.
These are some of the more common things to look out for when you're avoiding thin shot in golf. If you understand the swing faults that cause it you'll have a far better chance of deciding which fault you have to correct and eliminate it for good.