A fat shot is a mis-hit golf shot where the club head strikes the ground before it strikes the ball.
The result of this is that the speed of the club head is transferred into the ground and by the time the club head reaches the ball, the ball will not be hit very far at all. The club head strikes the ground before the ball because the lowest point of its arc is reached too early, rather than it being reached where the ball actually is.
There are a variety of reasons why this can happen, such as incorrect weight shift on your backswing and/or downswing, early club release into impact, but whatever the cause, it can be corrected simply by moving the low point of the swing arc to where the ball is.
In order to do this, work on the following drill. Place a head cover on the target line to the right of the golf ball, so that it is opposite the outside of your right foot (for right handed golfers). Take your set up position and hover your golf club up off the floor. Swing the club back so that you do not knock the head cover out of the way.
On your downswing, work on hitting the ground with the club head to the left of the head cover. If your low point for your swing arc is still too early, you will hit the head cover. Work on swinging the club head down over the head cover and then strike the ground. Your low point will now move to where the ball is. Do this slowly at first to get the low point correct without hitting the head cover and gradually speed your movement up. Once you are feeling confident with the new movement, introduce a golf ball and begin to hit balls, but still use the head cover to provide you with the information about where the low point of your swing arc is.
Working on improving this will really improve your ball striking and you will also see an increase in the distance that you can hit the ball as all of your club head speed is now being delivered to the golf ball.