Nada is the story of two families, both of whom belonged to the same socio-economic class before the war, but because of the senseless brutality of the Spanish Civil War, the culture is in shambles, which means for the protagonist, Andrea, that life will never be the same. Her parents now dead, she finds herself sustained only by the support of a totalitarian, oppressive government that was partially responsible for her current state. Her friend Ena however, retained her wealth.
In this way, it could be said that the central idea of the novel is about picking up the pieces. The title is reminiscent of that theme, that nothing was left from the old age. This is also an adequate description of why existentialism was the logical conclusion for the victims of the Spanish Civil War, and also after WWII, just a few years later. Europe was war-torn, and for the first time in the history of man, violence became industrialized through the military, allowing for brutal visions of warfare that the eyes of man had never seen before. Andrea exists in the aftermath of that jarring, disenfranchising paradigm shift. So this book is also an accidental commentary on the development of existentialism in Europe.
Another important layer to this existential reading of the novel is that, like Camus' L'etranger, the central characters seem to be in a state of post-trauma, or shell-shock. Many family members live together in a home, but no one cleans. No one tends to the house. No one bothers. It's as if they're still trapped in the horror of their situation, unable to see a path forward. It's because of Andrea's calm, detached demeanor that she finds her own escape from Barcelona. This makes her a kind of anti-hero, and in fact, some have argued that an androgynistic reading of the novel might be in order. In other words, Andrea represents a type of genderlessness because of her abandonment of social constructions to help guide her sense of self. This is truly existential.
