This is the Hering Illusion!
Click anywhere on the animation to take control (then click again to relinquish your iron grip).
The straight red lines should appear to be bent when they are close enough to the centre of the radial lines. If you wait, the red lines will change into a red square (not because of the illusion - I just programmed a square to replace them).
Learn more about the illusion here on the Illusions Index.