The Members' Lobby is the room outside the entrance to the Commons Chamber. Only MPs and lobby correspondents have access to the Members' Lobby while the House is sitting. The Lobby contains the Members' message board, for urgent telephone and written messages, and the bomb-damaged Churchill Arch which acts as as a memorial to the bombing of the Chamber during World War II.
Further information on this subject can be found from the following links.
About Parliament: Layout of the Palace of Westminster
Panoramas of Parliament: Members' Lobby