Flexing wings means that when the plane is stopped on the ground, the wings are curved down (because of gravity) and when the plane is flying, the wings move are get curved up, because of the lift. It's an animation that was simulated on some FS2004 planes, and in the default FSX liners too. The wings are now flexible, they can blend depending on the aircraft behaviour, juste like what you can see from the windows of a real plane
Unfortunately, on some video cards that are too old and that do not support all of the DX9 features (correct me if I'm wrong) this animation does not work, and instead of a flexing wing, the player would see a wing that sperates into several parts, a very weird effect.
