Kundera uses misunderstood words to show a little more clearly why the characters had the types of interactions they had in the novels. It seems that Kundera is trying to put focus on how two different lifestyles could alter the meanings of otherwise simple-to-define words. Rather, each character has different connotations when it comes to words, straying from their intended denotation. Using her musical style of writing, it can be explained that when people meet up too late in life, their life's "compositions" are already written, so their motifs don't harmonize. This is demonstrated through the relationships between Tomas and Tereza, working in harmony, and Franz and Sabina, working in discord.
Focusing on the relationship between Franz and Sabina, one word 'woman' has a separate meaning for them. While both are equally valid interpretations of the word, they have lived life differently, causing them to have their predispositions about the word. Franz thinks of the word and pictures motherhood. Defining a person based on their gender, giving that women are the child bearers. Franz sees Sabina as a woman, but she refuses his definition. She insists that she is an entity first, a woman secondly. In other words, she is asserting the fact that she has more meaning to life than to just be a woman.
Fidelity and betrayal is another one of these misunderstood words between the interaction between Franz and Sabina. With Franz, he is an advocate of being faithful and fidelity in general. He despises himself for being unfaithful to his wife to be with Sabina. However, in Sabina's case, she's used to a life of betrayal. She finds the concept to be intriguing on a physically exciting level. In all of Franz's guilt, he ends up allowing his wife to know his secret, in order to reestablish that sense of fidelity. She responds as if she doesn't care, but the relief he felt afterwards was easily picked up on.