Acceleration is an important concept related to the golf swing. We must accelerate the club through the golf ball at impact to hit it farther and straighter. Decelerating through the golf ball will cause problems and lead to inconsistency, poor contact and compression of the ball, and more errant mis-hits.
How to think about acceleration:
The golf swing should gradually pick up speed so that the club head is moving the fastest through impact. The club needs to pick up speed and build momentum so this can happen. If you swing too fast at the wrong times, it will not be possible to accelerate through the ball efficiently and effectively.
A common error would be having a backswing that is too fast. Then the club slows down on the downswing because it will lose speed. You cannot keep up a downswing that is faster than the backswing if the backswing is too fast. It just won't work out well.
Another common issue is swinging too quickly from the top of the backswing/start of the downswing. This will also lead to the club head slowing down through impact.
If you want the club to be traveling the fastest at impact, try to feel that about halfway in your downswing is where the club will start speeding up. Don't try to speed up in the backswing or from the start of the downswing. Make those parts of the swing smooth and the momentum and accleration of the swing will gradually build so you can be swinging the fastest through the golf ball.
Remember that you don't want to swing so quickly through impact that you lose the feeling of where your club is. That is a cue that you are swinging too fast. You should always have the feeling of where your club is at all times.
Also, don't swing so slowly in the backswing that you are steering the club. You need to have a little flow, momentum, and smoothness to the backswing.
A good backswing leads to a good downswing which leads to better impact and hopefully good acceleration through the golf ball.