This probably refers to Oedipus the King, the second of the ancient Greek playwright Sophocles’ three Theban plays (between Antigone and Oedipus at Colonus) and the most highly regarded.
On the surface, the play appears in the story here as a reminder to Tereza of her beloved Tomas, because he had given her a copy early in their relationship, which helps to calm her as she prepares to have sex with a relative stranger -- the engineer who has been trying to pick her up at the bar where she works. Seeing the familiar book “made her feel as though Tomas had purposely left a trace, a message that her presence here was his doing”; that, as the narrator said a page or two before, “she was actually being sent to him [the engineer] by Tomas.”
Of course, Sophocles’ play deals directly with themes of fate and free will (which relates to Beethoven’s “Es muss sein!” and Tereza’s notion that she is going to bed with the engineer in accordance with Tomas’s wishes) and state control (another huge theme in Unbearable Lightness).
Finally, at the start of Part Five, “Lightness and Weight” (pages 175-176 in this edition), the narrator reminds us that Tomas gave Tereza a copy of the play at the beginning of their relationship, and tells the story of Oedipus in more detail because Tomas will use the myth as the basis for his essay about the culpability of people under a Communist dictatorship -- the essay that will cause him to lose his job as a surgeon in a Prague hospital.