In The Unbearable Lightness of Being, Milan Kundera uses lightness and heaviness as metaphors for two different philosophies of life, expressed by Tomas and Sabina and by Tereza and Frantz, respectively. Let's look at this in more detail.

Lightness refers to a way of life that takes nothing especially seriously, that reaches out for pleasure rather than meaning and for passion and sensory experiences rather than intellectual and rational consideration. Tomas lives this kind of life through his many affairs, even after he marries Tereza. He claims that his sexuality is separate from his love for her, that it is light and free. Sabina, too, holds this kind of philosophy in her continual seeking for new experiences of the unknown.

Heaviness, on the other hand, is all about discovering meaning in life. It focuses on a search for something more than mere pleasure or passion or the senses, something deeper and much more profound. Tereza loves Tomas, but she is seriously hurt by his infidelity because she seeks meaning in her marriage and a connection that is deep and permanent. She deliberately returns to Prague even though it is dangerous because she has discovered meaning there, and she must strive for it even if Tomas is not willing to follow. Franz also pursues the heaviness of life. While he has an affair with Sabina, the guilt of betraying his wife always follows him. He wants to know the meaning of life, yet he fails to find it in Sabina.