>>9944129Imagine a particle, lets call it brainlet, sitting at the center of pic related. All of the particles in the image are randomly moving around and bumping into eachother (temperature), the probability of brainlet being hit by the left side is much greater than the right side (because there are more particles there) so brainlet will be pushed to the right. This applies to all the particles so they will diffuse to the right.
That being said, brainlet cold perfectly be hit in just the right way by particles to his right and move to the left, but the chances are much, much lower, so in agregste that happening to all particles is virtually impossible.