What is the “panopticon”? How is visibility a “trap”? How does the panopticon work to “induce in the inmate a state of consciousness and permanent visibility that assures the automatic functioning of power”?

What is the “panopticon”? How is visibility a “trap”? How does the panopticon work to “induce in the inmate a state of consciousness and permanent visibility that assures the automatic functioning of power”?