The flex point is the position of the shaft that the shaft bends or recovers from while the club is accelerating toward the ball.
LOW flex point shafts produce higher trajectory shots. Since the ball travels farther the longer it is in the air, people who are looking for better distance will choose LOW flex points. Higher handicap golfers seem to prefer LOW flex points as well because the higher lofts will have more back spin and less side spin, resulting in straighter ball flight.
HIGH flex point shafts propel the ball lower. Stronger golfers and those with lower handicaps tend to prefer the HIGH flex points because the lower trajectory will control the adverse effects of sidespin.