Flex refers to the amount of bend that a shaft has. A more flexible shaft will help the club "whip" through the ball, creating greater clubhead speed and more distance. This works well for golfers with slower swing speeds. Golfers that swing the club hard and fast will lose accuracy if the shaft is too flexible and should get a stiffer shaft
To determine what is best for you, look at the chart below
If your distance falls right at the cut-off for one flex or the other you have two options.