A lobby is a space where many people come together to communicate with each other. The term can be compared to a hotel: the lobby of a hotel is the central arrival, stay and departure room. From there you can enter other rooms.
In the sense of Settlers 3, the lobby is the room where people meet and from which they can enter open games. This is fulfilled by the aLobby.
In colloquial language, sometimes the Minichat before a game is called a lobby. However, if you look at the word origin (see below), this cannot be true.
See: https://de.wikipedia.org/wiki/Lobby_(Raum)