[tzar]

It's an analogy for the second law of thermodynamics: https://en.wikipedia.org/wiki/Second_law_of_thermodynamics. The point is that it's “challenging” and that complex events “evade a straightforward explanation”. That “challenge” is our intuitive experience of the directional arrow of entropy. A similar analogy would be to consider the difficulty of un-cracking an egg. Also, imagine comparing one bowl of soup to another bowl which was made just from tasting (or distilling) the first. Intuitively these are not likely to taste the same, otherwise a notable episode of Seinfeld would be invalidated.