The phrase represents the impossibility of her connection with the postmaster. Ratan is haunted by these words not only because they are a kind of rejection, but also because they prove that the postmaster has not even considered the fact that she might want to stay with him—or that she might have feelings for him.
The postmaster is concerned only with his own comfort—perhaps because he considers himself above the country folk of the village. Ratan is useful to him only in that she is able in small ways to give him some of that comfort. Conversely, being of use to the postmaster is what gives Ratan's life meaning. What is insignificant to the postmaster (e.g., idle chat about her life or teaching her how to read) is for Ratan a tacit recognition of her personhood.
Thus, when the postmaster responds to her request with "What an idea," it becomes clear to her that she does not matter to him. The story ends with Ratan in tears, but also suggests that there is little else for her to do:
False hope is clung to with all one's might and main, till a day comes when it has sucked the heart dry and it forcibly breaks through its bonds and departs. After that comes the misery of awakening, and then once again the longing to get back into the maze of the same mistakes.
These words are spoken by the postmaster in answer to Ratan's question about possibly accompanying her employer back to Calcutta.
In the text, we learn that the postmaster's answer haunts Ratan "that whole night, in her waking and in her dreams." This is because his answer is an indication of his intentions, and as time progresses, Ratan's fears are confirmed when the postmaster prepares to leave without her. To Ratan, the postmaster's insensitive answer is also devastating proof that he doesn't view his relationship with her in the same light that she does. While Ratan has begun to think of the postmaster as family, he still remains emotionally detached from her.
Even on the day of the postmaster's departure, Ratan holds out hope that he will change his mind. However, he never does, and she is left devastated by his seemingly callous rejection. Rabindranath Tagore maintains that our "foolish human nature" causes us to hope beyond reason; this is why the postmaster's answer haunts Ratan. The truth of reality is sometimes incongruous with the perceived desires of the heart, and in Ratan's case, the contrast is devastating.