This optical illusion is a classic example of how our perception of reality can be distorted. It consists of a grid of alternating black and white squares, arranged in a way that creates the illusion of a set of feet walking across the grid. The illusion is created by the alternating light and dark squares
From Wikipedia
The stepping feet illusion is a motion perception phenomenon involving two "buses," one blue and one yellow, moving horizontally across a "street" consisting of black and white stripes. Although both of the buses move at a constant speed, their perceived speed varies dramatically.