That is a good question and one that is often misunderstood.
Clubhead speed is how fast the head of your club is traveling at its peak velocity. If you have good mechanics, this should be how fast your club head is traveling when it strikes the ball.
Club head speed is one part (maybe the most important part) of the equation that determines how far you hit the golf ball. The other factors include whether you hit the sweet spot, the loft of the club, direction of the club, etc.