To understand After World, you have to understand what came before it.
Due to a solar storm caused by Mercury plunging into the sun the entire Earth is cast into a blackout in which nothing running on electricity works. Thus dawns the Red Sun Millennia. It is over a thousand years before that sun sets and the yellow one reappears. During the Red Sun Millennia those people who were weak, unfit for survival, or just could not compete enough for resources died off. The survivors were a hardier stock of people. Any advantage that allowed someone to survive a little better than others became part of the genome.
The rising of the yellow sun again marked the start of After World. A dozen or more of distinct races of humans emerged each with their own advantage be it strength, size, speed, or fur.
A Life in After World focuses on one of the lives in this new place with new people and tells their life story.