Instanced maps are maps created especially for you and your team. These maps are not available to players who are not a part of your group.
Instanced maps that have no players on them are closed after a short time. If you log out or are disconnected from an instanced map, you will be returned to your last position on the map if it is still open when you reconnect. If the map has been closed, you are returned to the last non-instanced map you occupied.
An instanced map will reset in about 30 minutes, alternatively, if you join a team the instance will reset.