The key to a great golf swing is balance throughout the movement and the only way to achieve balance is to have the correct base to move from.
During your golf swing, your shoulders are making a huge rotational movement. Your shoulders are the widest part of your body, so it is absolutely vital that the base underneath your shoulders is able to cope with the rotational movement that is happening above it. If the base is too narrow, you will lose balance and if the base is too wide you will become too static and not be able to move correctly.
To achieve the correct base to make a golf swing from, you should generally work on achieving a stance that is shoulder width apart. If you are not sure how wide that actually is, then place your golf club across your shoulders. Hold the end of the handle to the edge of your shoulder and then place your thumb and index finger on the shaft to represent where the outer edge of your other shoulder is.
This distance, from the end of the handle to where your thumb and finger are should be the distance between your big toes when you take your stance up, so place it on the floor and have a look - adjust your feet width if necessary.
Now you have your base as wide as the movement above it, you should notice an improvement in your balance throughout your golf swing and you should see that you are hitting much better golf shots.