What is Foucault's theory of panopticism?

Foucault's theory revolves around the rise of the surveillance society. He based his theory on the design of the panopticon, which is a prison that has cells all around and a tower for the guard in the middle with a beaming light. What this means is that the guards in the tower can see all the prisoners but the prisoners cannot see them. This will alter the behaviour of the prisoners, as they constantly have the feeling that they are being watched (even though there might not be a guard in the tower). Foucault uses the example of the panopticon to argue that technological advances such as CCTV changes people's behaviour in everyday life. Our everyday actions become more and more monitored through the feeling of being watched.