I haven't read it thoroughly, but generally it might involve the complex world - building of the Code Geass universe, perhaps the backstories of some characters, and the early events that set the stage for the main story in the Code Geass series.
Well, this light novel probably expands on the world of Code Geass. It might introduce new characters that are somehow related to the main plot but haven't been fully explored in the anime. It could also detail the early days of the conflict between Britannia and the other nations. The story may be filled with political intrigue, personal vendettas, and the discovery of the mysterious powers that play a crucial role in the Code Geass saga.
The 'Code Geass Stage 0 Entrance Light Novel' likely delves into the origin stories of the main characters. It could show how Lelouch got his initial ideas or how the power dynamics in the world of Code Geass started to form. Maybe it also explores the political situation that was brewing before the events of the main anime series. It's a chance to understand the roots of the story on a deeper level.