If you zoned out in class when your physics teacher brought out the bell jar, that's OK. In fact, it might be useful just to imagine what a bell jar is. Perhaps a jar shaped like a bell? Shaped like the Liberty Bell in Philadelphia, maybe? Certainly an unusual shape for a jar. What would they keep in a jar like that? Something equally unusual?
Well, you're not far wrong. A bell jar is – surprise! – shaped like an upside-down bell and creates a vacuum effect that comes in handy when you're trying to preserve plants or perform physics experiments in the classroom (like this "phunny" guy). The peculiar feature of the bell jar is that it keeps everything inside hermetically sealed from the outside world. Whatever's inside remains preserved, static, unchanging.
In The Bell Jar, the main character uses the bell jar as the primary metaphor for feelings of confinement and entrapment. She feels that she's stuck in her own head, spinning around the same thoughts of self-doubt and dejection, over and over again, with no hope of escape. But she also uses the bell jar as a metaphor for society at large, for the way that people can be trapped inside stale social conventions and expectations.