The yellow bench represents for Tereza one of the fateful signs that she and Tomas were meant to be together. She finds him sitting on it, waiting for his train, a day after she had sat on it with a book. The “six fortuities” like the yellow bench give her the courage to follow Tomas to Prague and seek him out.
Much later, after many years of unhappy marriage to the philandering Tomas, but shortly after having the Petrin Hill dream, sleeping with the engineer, and traveling to the spa town with all the Russian names, Tereza is so unhappy, “She wanted to die.” She takes a walk and sees red, yellow, and blue park benches being swept down the Vltava River (section 29 of Part Four, pages 170-171). This may be a dream, or it may refer to an actual event; it is hard to tell.
But one might say that dream or not, it symbolizes the washing-away of Tomas and Tereza’s unhappy life, including the fascination for fortuities like the yellow bench in her hometown where she had met him.