These platforms use the Sine behaviour, which automatically adjusts a specified property (position, opacity, angle, etc.) along a wave. The most common use for this is adjusting the position, which will make the platform bob up and down or sideways (depending on what you choose). That platform is then also assigned the solid behaviour to make it an object that the player can walk on.
You can see these in action in the platformer template. I'd suggest that you open up the template and play around with all the sine behaviour properties. You can do a hell of a lot with this behaviour.