"Die Lebenswelt" is a German term that means "the lifeworld" or "the world of life." "Die Lebenswelt" refers to the everyday, concrete reality of life, or the lived world of ordinary human existence, according to philosophers like Edmund Husserl and his student Martin Heidegger. 
It encompasses the subjective and immediate aspects of life, including personal histories, work, family life, and the world as it is perceived by individuals. "Die Lebenswelt" emphasizes the rich and complex tapestry of human existence, juxtaposing abstract or theoretical perspectives. Husserl is critical of the sciences for neglecting this lifeworld in favour of scientific and mathematical investigations in the passage you gave from Milan Kundera.
 
In the context of phenomenology, a philosophical approach that focuses on conscious experience, "Die Lebenswelt" expresses the lived experience as the foundation for understanding the world. It defines the idea that our consciousness is always embedded in a specific lifeworld, and our understanding of reality is shaped by our interactions within it.