The phrase "to close the door" means to bring an activity, situation or opportunity to a conclusion. It can refer to ending something that has been ongoing for some time and making it no longer possible to continue with it. For example, if someone says they are going to close the door on their past, it means they have decided not to go back to certain experiences or relationships from their past and instead focus on moving forward. In a more literal sense, "to close the door" can refer to physically closing an actual door by shutting it so that no one else can enter or exit through it.