A lét elviselhetetlen könnyűsége c. könyv címe mire utal?

A könyvben benne van. Itt angolul olvashatsz a dologról:

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 much significance for humans.

In the novel, Nietzsche's concept is attached to an interpretation of the German adage Einmal ist keinmal ('one occurrence is not significant'), namely an "all-or-nothing" cognitive distortion that Tomáš must overcome in his hero's journey. He initially believes "If we only have one life to live, we might as well not have lived at all," and specifically (with respect to committing to Tereza) "There is no means of testing which decision is better, because there is no basis for comparison." The novel resolves this question decisively that such a commitment is in fact possible and desirable.

De mint mondtam a könyvből is kiderül.