A lobby is another word for a room that serves as a meeting place. There are several lobbies in the Palace of Westminster which are used by different groups of people. The Central Lobby is situated in the middle of the Palace of Westminster and forms the crossroads of the building. Everything to the south of it is part of the House of Lords and connects up to the Peers Lobby which is directly outside of the House of Lords chamber. Everything to the north is part of the House of Commons and connects up to the Members' Lobby which is directly outside of the House of Commons chamber. The Members' Lobby is the room next to the House of Commons Chamber. It is a place where MPs can collect messages and where the Chief Whips' offices are located. The Peers' Lobby is next to the House of Lords Chamber. When the House of Lords is sitting it is the focus of activity where members can meet and discuss forthcoming business.
Further information on this subject can be found from the following links.
The Central Lobby virtual tour
Related glossary term: Aye and No Lobbies
Related glossary term: Contents and Not Contents Lobbies