I've been pondering on that quite a bit lately.  While I do intend this to be a love letter to Witcher fandom, I also want it to be pretty accessible.  Very likely, what I'm going to do is both offer up sources for lore (wikis, etc.) as well as highlight/summarize the lore that will be vital within the OOC post.  

Beyond that, I'll likely have my NPCs explain situations and events as they arise.  Afterall, the characters are going to be learning about how all this works, too, in-game.

I suppose the short answer is: Prior knowledge is a plus, especially if you intend to branch off from the linear quest for character development/explorative purposes.  However, if you don't have prior knowledge, I will be providing the knowledge you need in one format or the other both at the very beginning and as we go along.

In the meantime, here is a broad introduction to the world of the Witcher that does a decent job of touching on a little bit of everything:

[center][youtube]https://youtu.be/EPg-0zHMP9s[/youtube][/center]