So what the fluctuation theorem tells us is that as this ratio between the forward and reverse rates of a process gets larger and larger, we expect more and more entropy to be produced as a result of this process. Let’s turn that around though, causally:* the reason that a process may be spontaneous in one direction and unlikely-approaching a limit of impossible in another is because of the direction of increasing entropy set by the arrow of time*. Energy is always dispersing because we live *after* the big bang, and if something is possible in one direction but not in reverse this fact is a result of the energy that the process disperses. Once dispersed, energy will not return to a more concentrated form without energy from some other portion of the universe consequently being dispersed in more than compensating amounts. Mathematically, irreversibility is an exponential function of entropy production so the fluctuation theorem can be rendered in a mixed math/plain language form like so:

(forward rate/reverse rate) = exp^[entropy change of the surroundings]