In short, Privacy means that you are not under surveillance. In this example, Bob wants to talk to Alice privately, so he shuts the door on the prying eyes of Jack. So that he can't hear their conversation.
Privacy is Bob and Alice's ability to seclude themselves from the awareness of others. It is the ability that Bob has, to close the door on Jack, so that he cannot see his actions anymore.
+When you are looking for Privacy, always ask yourself : Privacy from what ? Privacy from whom ? in other words, Whom do i want to close the curtain on ?