The title, The Bell Jar, is extremely relevant to the book. Throughout the story, Esther feels as though the bell jar of her depression has encased her, and she will never escape.
This quote is from the very end of the novel, when Esther's depression has reached its pinnacle. She is trying to explain how her sickness feels to her mother, who believes that Esther's illness is something you can just wake up from. She explains that it is an inescapable bell jar, and she is trapped.