Kundera uses the metaphor of a music composition to describe an individual's life. To go through life is to slowly compose our own music. And, like any good composition, the music of our lives contains motifs. In music, a motif is a section of notes that is repeated throughout an otherwise varied composition. To extend Kundera's metaphor, a motif in a person's life is an object or idea that is repeated in different ways. The example he uses is the bowler hat for Sabina – an object that crops up repeatedly, always with slightly different meaning.
Two interesting discussions branch off of this metaphor. The first explores what happens when two different people try to share their lives. "While people are fairly young and the musical composition of their lives is still in its opening bars," explains the narrator, "they can go about writing it together and exchange motifs (the way Tomas and Sabina exchanged the motif of the bowler hat), but if they meet when they are older, like Franz and Sabina, their musical com-positions are more or less complete, and every motif, every object, every word means something different to each of them" (3.2.13).
The second point has to do with our ability to recognize motifs – or literary elements – in our own lives. The narrator argues that human lives are in fact composed with the same degree of artistry as a novel. They are symmetrical and crafted; they have motifs and symbols; they are composed with an attention to aesthetics:
[Human lives] are composed like music. Guided by his sense of beauty, an individual transforms a fortuitous occurrence (Beethoven's music, death under a train) into a motif, which then assumes a permanent place in the composition of the individual's life. […] Without realizing it, the individual com-poses his life according to the laws of beauty even in times of greatest distress. […] It is right to chide man for being blind to such coincidences in his daily life. For he thereby deprives his life of a dimension of beauty. (2.11.4-5)
This has interesting implications when we think about the narrative trickery going on in The Unbearable Lightness of Being. The narrator openly admits that his characters are fictional, and that the story is intentionally built with symbols (like the bowler hat) that are supposed to mean something. But this passage argues that such fiction doesn't make the characters or their story any less representative of real life. Because real life looks like fiction, fiction can look like real life even when it admits to be fiction. Make sense?