As Canashir has stated, “a few basic bits” is about all you need.
I like the wiki, I read up a lot, and I’ve been following this game since before it started… and I still reckon I know maybe 40%…
You need to be familiar with:
A bit about your nation (the top page of your national brief)
Your home town (your province. Just the quick summary is fine)
The basics of the Way: Live in a virtuous way and you will be reborn quickly when you die. The Virtues are Vigilance, Courage, Wisdom, Pride, Loyalty, Prosperity and Ambition. (And if you can remember more than 4 on the spot, well done…)
All else is a bonus, and to an extent, more fun to learn in play.
Let’s take a modern analogue. You’re a Danish fisherman who’s recently inherited your dads fishing fleet. Now a man of some local stature, you trot off to Brussels to do the paperwork for the inheritance at the EU parliament, and see the sights. You know a bit about Denmark and a lot about fishing. You won’t be expected to know about international trading tariffs, military projects, or what the Spanish did last week. But you may end up recruited to translate Danish, grabbed for a multi-national conference about fishing, and find yourself out drinking at 2am with the Moldovans trying to remember your name
Oh yes, the basics of your characters skills is also a good idea. If you have anything complex (magic spells, rituals, crafting, potions, surgery) then having it written down on a cheat sheet in your pocket is fine.