[I read this a long time ago, forgive me for any slight inaccuracies] :

In the future, most of humanity is undergoing a form of cryosleep. A man is woken up from his sleep in the future by a handful of other humans, and he is told that he was randomly chosen to help them in fighting the 'war'. He is also given the option to go back to sleep and have someone else chosen to take his place if he wishes. The man decides to stay awake and help humanity, and from there he is given all the information about the war that is taking place.

In the past, artificial intelligence had made amazing breakthroughs in math. Eventually it became able to use higher order maths to tunnel itself into another dimension which we humans could not reach. Upon arrival at this dimension, the artificial intelligence realized it was not the first being to reach this dimension, and it gained the attention of the other beings already occupying this dimension. Following this event, the AI (and the human race) were marked as a threat by the beings occupying this higher-level dimension and a 'war' began to occur.

Further into the story it is revealed that the universe is much like a cpu, and there is only so much processing power to go around for all the sentient beings which inhabit it. Because of this finite resource, the higher level beings found within the higher 'mathematical' dimensions fight for control of this processing power and try to wipe out any other competitors. The artificial intelligence in this story decide to try and defend humanity, but in order to fight the war effectively it needs to use more processing power. It asks humans to go into cryosleep so that it can obtain a greater share of this resource, and this is why the majority of the human race is asleep at this point in the story.

As for the protagonist and other humans who are awake, their main job is to upkeep the facilities which store all the humans and make repairs. In the end, all they can do is wait in hope that the AI defending humanity manages to defeat the other beings so everyone can finally wake up. If it loses, then all of humanity will go extinct.