Selecting a different flex option allows you to choose whether your board will have a softer or stiffer flex as opposed to the board's usual flex pattern. A flexier board can be more comfortable for cruising and more playful when it comes to dancing, while a stiffer board is better for freestyling and high speed shredding. Remember, everybody's preference is different, so you should get what feels right for you!
The soft flex is intended to accomodate riders weighing 140lbs or less, although it is still strong enough to support heavier riders looking for a super flexy deck.
The hard flex is intended to accomodate riders weighing 210lbs or more, although it is still ridable for lighter riders looking for a super stiff deck.