A "fat shot" is a term used to describe a shot that travels less distance than expected due to a swing that takes a large divot behind the ball. When a player hits it fat, it usually means that he failed to strike down on the ball. This results in the ground that is being displaced pushing the ball forward, rather than the club face striking the ball cleanly.
A fat shot means that a player is striking the ground too far back in the swing process. In an ideal swing, the club strikes the ground in front of the ball, after contact, creating a divot in front of where the ball was resting. Most often, a shot comes up fat because a player has failed to get his weight forward before the moment of impact. By keeping the weight on the back leg, instead of transferring it onto the lead leg, the player is leaning back at impact, causing him to hit too far behind the ball.
To get proper impact, the first motion in the downswing should be a slight flexing of the legs, lowering the center of gravity. The player should then move parallel to the ground toward the lead leg, so that the player's center of gravity is slightly forward of the ball when contact is made. At the moment of impact, an ideal position for the player's body will see her lead leg and her lead arm creating a straight line up from the ground. By having the hands forward like this, the shaft is angling back towards the ball, allowing the player to drive down on the ball without hitting the soil behind it.
A great drill to practice attaining a proper finish is to slowly work your way up to it. Begin by standing over the ball in the ideal contact position, with the straight line from your lead leg through your lead wrist and arm, and your trailing wrist breaking so that the club is ready to hit where a ball would be. Bring the club back slightly and swing forward, being sure you maintain this form at the bottom of the swing. Continue to gradually increase the length of the arc of the swing until you are taking a full swing with the proper drive down on the ball.