Philosophical Underpinnings
Challenging Friedrich Nietzsche’s concept of eternal recurrence (the idea that the universe and its events have already occurred and will recur ad infinitum), the story’s thematic meditations posit the alternative; that each person has only one life to live, and that which occurs in life occurs only once and never again — thus the “lightness” of being. In contrast, the concept of eternal recurrence imposes a “heaviness” on our lives and on the decisions we make (to borrow from Nietzsche's metaphor, it gives them "weight".) Nietzsche believed this heaviness could be either a tremendous burden or great benefit depending on the individual's perspective.
The "unbearable lightness" in the title also refers to the lightness of love and sex, which are themes of the novel. Kundera portrays love as fleeting, haphazard and perhaps based on endless strings of coincidences, despite holding such significance for humans.
Read more about this topic: The Unbearable Lightness Of Being