Esther, the narrator of the novel, is disillusioned with the stagnancy of her life and feeling trapped by society, as though she is trapped under a bell jar. These feelings are prevalent throughout the novel - as the people around her are able to peer in and see her magnified under this glass display case; yet, she is unable to communicate back with them. She is trapped, on display and festering in her own stenches beneath her glass prison. Her mental degradation is on display for all of society to see, however there is an invisible barrier preventing her from calling for help, and preventing people from helping her in turn.
This sense of isolation and being watched and judged by society is found in some of Plath's poetry as well.
Quotes from The Bell Jar:
“...it wouldn't have made one scrap of difference to me, because wherever I sat - on the deck of a ship or at a street café in Paris or Bangkok - I would be sitting under the same glass bell jar, stewing in my own sour air.”
“To the person in the bell jar, blank and stopped as a dead baby, the world itself is a bad dream.”
cf. Plath's poem: Tulips.