For my homebrew world, I wrote a two page document covering:

• Geography: the rough layout of their starting city (the main districts and well-known landmarks), the main species that can be seen around the city (they're not limited to playing these, but I think it's helpful if you're going to play a dragonborn to know whether you're an outsider in this city or not), and a high-level sentence each on the handful of main locations that can be easily travelled to from the city - i.e. the stuff that any resident should know.

• Magic and religion: the pantheon of major gods and their domains, and a line noting there are other religious/magical traditions but most residents of the city would be unfamiliar with them.

• Politics: how the city is governed, who the noble houses are, and what reputation is widely known about each of them (not all of these are deserved...)

• History: some very high-level history of the city and world (at the level that an average resident would know - the equivalent of the name of this world's Roman Empire who founded the city, and what happened to them).

I figured that two pages is short enough for anyone to read and get some sense of the world I'm dropping them in, without going into so much detail that it takes away their ability to explore the world (which is dramatically bigger than the city).

There's also no real cost to people not remembering what's in the two-pager - people can get away with assuming it's a fairly generic fantasy world at first and I can easily resupply key info while DMing - but I figure most players like to know a bit about the game world before they start.