[This is a short explanation, intended for non-scientists, of the main concepts in Isaac Asimov's "The Last Question".]
In thermodynamics we learn that in order for an engine to work, energy must flow, from one place to another. For that to happen, there must be a physical difference between the two places, for example, in temperature or pressure. Negative entropy is a measure of the amount of "difference" that exists in a system, for example, the universe. Entropy is therefore a measure of homogeneity. If entropy is at its maximum, there are no differences, and no engines can work. That includes the engines that run the cells in the body.
The Second Law of Thermodynamics states that in any closed system, where entropy is not already at its maximim, for example, presumably the universe, the entropy must increase. We are destined to encounter the Heat Death of the Universe.
Since the work of Claude Shannon, it has been clear that there is an intimate relationship between entropy and information. Basically, the higher the entropy of a system, the lower the amount of information it has. At maximum entropy, there is no information. This is especially bad for computers, and conscious entities, both of which are critically dependent on the availability of information.
Asimov's story, "The Last Question", deals with the tragedy of the entropy problem. He solves the problem by the introduction of a God-like computer, AC, that learns how to reverse entropy. AC creates a program that will do this, and activates it with the command: "Let There Be Light." (Before the introduction of windows systems, computer programs were generally activated by typing their names on the computer console.)
It is most important that in order for this to happen, AC itself must be immune from the effects of the Second Law. To accomplish this, Asimov uses a common science fiction literary device: hyperspace. In hyperspace, all sorts of special things can happen, in this case immunity to the Second Law.
Now, the command itself. Classical thermodynamics assumes nothing about the substrate that gives rise to its concepts. Most importantly, the idea that in matter, heat is manifest as the motion of myriad particles, is not a part of Classical Thermodynamics. (It is amazing how much can be accomplished without reference to atomic theory.) The particles are introduced in what is called Statistical Mechanics. The basic of this idea is that thermodynamic effects are the result of the random motion of myriad particles. Low entropy ensues when different regions have different random properties. A commonly used example is half a glass of cold water and half a glass of hot water. Mix them together and you get a glass of warm water. The same total energy; but the entropy has increased, and there is no longer a temperature difference to run an engine. The only way to restore the original state is to introduce energy from outside the system, if the system has an outside.
But wait; I lied; it is not quite the only way. The other way is to wait, and to do nothing. As I mentioned above, the motion of the particles is random. For example, in its random motion, all the air in the room could pile up on the other side of the room, leaving me here to suffocate. What is the probability of this? 1/(very large number). How large is the number? If you tried to write the number on paper in the usual manner, the universe could not begin to contain the paper, even if each digit was the size of a proton. (See also: "Theological Exponentials".)
So after the heat death of the universe, by virtue of the random motion of particles, the universe might compress itself back to a tiny speck, whereupon another Big Bang might occur. But we may have to wait a while. But then, in the face of infinite death, we can wait. On the other hand, it is likely that when the particles get close, there are interactions which cause substantial deviation from the simple random model, so a whole different story is required. Maybe the universe can never recompress to a tiny patricle. Do you believe that there are things that only happen once? In any case, perhaps the program that AC activated with "Let There Be Light" is a way to speed up or otherwise enable repetition.
This is my conjecture; I do not know what Asimov had in mind. I would like to hear other suggestions.