This Mini-suppliment of 'Curse of Strahd: Rebitten' covers session zero.
It isn't required to play D&D and with the right group isn't necessary at all.
Simply, this is the gaming session before session one.
This is the first session where the players and DM will meet up and break the ice, discuss the campaign, create characters and backstories together, discuss schedules, house rules and lines that shouldn’t be crossed. Some people refer to this as a social contract, a spoken, unofficial agreement to play the game a particular way, while respecting each other.
This can be done in person or virtually but the power of doing this in person should not be underestimated.
In my experience running many different campaigns over the years I think it is better to run a session zero before the campain starts, to reduce the amount of future disagreements within the group and to make successful long running campaign that everyone enjoys.
- from the publisher's blurb