Snowboards have two types of flexibility: torsional flex and longitudinal flex. Torsinal flex is the flexibility across the width of a snowboard. Longitudinal flex is the flexibility from tip to tail.
A soft, flexible snowboard is great for beginners since it will turn more quickly and easier at low speeds. It requires less technique and strength as well. Freestyle snowboarders need more flexible snowboards as well for more board control and maneuverability. Freeriders and carvers need stiffer snowboards which are better on runs and for high speeds.