If the golf club is too high at the moment of impact, and hits the top of the ball it will roll along the ground, instead of flying through the air.
To avoid topping the ball in this manner, the first and most important thing is to make sure you set up with your body at the right height.  To check that your posture is correct, set up and then make sure that the club can't reach the ground a few inches behind the ball. During the swing, your body should stay at the same height from the set-up position right through to the top of the back swing. This ensures that you swing the club down through the ball, making the ball fly from the loft of the club.
One of the common errors in golf is topping the ball. However, if you follow these tips and practice the proper set up, you can avoid it.