The narrator reminds you that, when Tereza first came to Prague, Tomas saw her as a child put in a basket and sent downstream to him.
This image became very important to Tomas.
It was in his mind when he picked up a translation of Sophocles' Oedipus https://www.shmoop.com/oedipus-the-king/.
The narrator reminds us of the story of Oedipus, in case we aren't up to speed on our Greek mythology: Oedipus was raised by King Polybus and his wife. He was told by a prophet that he would kill his father and sleep with his mother, so he left home and went to Thebes. On the road, he quarreled with and killed a man. In Thebes, he married the newly widowed Queen. Only later did he discover that Polybus was his adopted father. The man he met on the road and killed was his real dad, and he's been married to his mother. When he realizes what he's done, he blinds himself and leaves town.