A new player sits at your DnD table...

...They've read the PHB and have a good grasp of the base rules including how to run their character. They ask "What do I need to know that isn't in the book?"... What do you tell them?

I am very thankful for the people that make up Gamer's Plane in accepting me here and easing my foray into PBP and TTRPG to an extent...as a newbie I have found many helpful people here that have helped me get into a game, learn it and have tolerated my weird quirks lol. That said, I know that a lot of newcomers are more informed in the system they are trying to play and would like to know more. What if the ''newbie'' asked the question above?

The rules are just guidelines for us all to have fun. They are not a weapon to beat each other over the head with. But that goes for any game.

I think this is really, at best, the session zero talk, right?

I'm by no means an expert on session zero, because, with the exception of PBP, I've been gaming with the same circles of ppl for about 11 years, so we all kind of already know where we all fall on this stuff... And with PBP I haven't been explicit about this stuff until fairly recently.

But as DM, I have started by telling them what I want out of the game. Which includes, as DM:
1. my gaming "style" (strict RAW, don't care about RAW/expect rulings on the fly and variability, somewhere in between, specific house rules, etc.)
2. my narrative style - which can include everything from:
A. the tones and themes of the campaign that I have in mind (low/high fantasy, politics and intrigue, investigation, horror, heists, etc.)
B. how "adult" the story might be (I default to the American movie-rating scale), before getting into specifics, which will also include...
(i) my and others' "Lines and Veils"
C. I'll talk about the setting and if there is some specific starting point for the plot (at this point I will ask for buy-in into a specific plot, or explain that it's a sandbox, or an open world, etc.)
D. Character creation, including any initial conceit or mechanical (alignment?) rules that there might be to ensure the party will be cohesive with each other and the setting

From there, or during that explanation of my wants, I'll ask them what they want out of the experience. Particularly, do all of those things that I laid out sound agreeable to them? And particularly, if the expectation is there, do they understand that the expectation is that the group works together, and cares about "the main quest"? Because right there, I think, is where most problems arise. When the DM has a campaign idea that they've planned, and they don't have much time for planning anything else, and they go into the game without expressing or getting an explicit understanding from the players that "This is the game." and the players just ignore hooks, or arbitrarily kill NPCs, or latch onto something in the periphery, and suddenly the DM is having a bad time, and so the players start having a bad time.

Bottom line: when ppl are there for different reasons than other ppl, problems start popping up. So make sure everyone is on the same page.

That said, many new ppl don't know what they want, because they don't know what the experience really has to offer, so you can guide them further along that line of thought. Which could include stuff like, I don't know...
1. is it a priority for them to develop a strong mechanical understanding of the rules? At which point you could spend more time explaining them in play, or not.
2. Is there a particular (narrative) character development arc that they want for their PC to go on, or are they just along for the ride?
3. How do they feel about character death?

For me, the point of D&D and RPGs as a hobby is not that the characters can go anywhere and do anything at any time, the point is to work together as individuals to make the story that is "on the table" into the best story possible, together. And I want players that want that. And so I will try to guide the table toward that experience.

It’s just a game.
Nobody cares if you get things wrong - there’s no money riding on this game.
The DM is the referee, but they’re not impartial as they’re also your cheerleader.
If your want to leave, then it’s cool - just remember to say goodbye.

To further Adam's points.
Assume you will get the first one wrong. If you do, don't worry about it, the next one will be less wrong. Don't stress about getting it right, you are not stuck with it, you can change it if you get it wrong.
The GM is there to help you. It is their job to be a fan of your character and to let them be as cool as they can. Ask for help, keep asking for help.

That last point is something that might be more important than we have realised. There is a safety tool called "Open Door", basically it says that "You can leave at any time. You are not committed to staying in a game that is not fun." It is most useful/used during con-games, but should be used a lot more in all internet Pick-Up-Games. Joining a PUG can be stressful, and being able to walk out if it does not work out is more important than people realise.
Defining an Open Door policy also provides a way to talk about "how you should leave". You don't have to justify yourself, but you should be polite and tell people (or just the GM) you are going. This last part is very important in PbP where vanishing without saying anything (ghosting) is all too common and people can end up waiting ages before realising you have left.

My Goal in the conversation I'm having with the player would be about creating a good character, because if the player gets bored of playing the character quickly and we don't want that to happen, it is Important to make sure they are not playing a character who is just Stats, as they probably will have trouble changing their character from stats to Personality.

Adults can take anything too seriously.

Sometimes I miss the unbridled imagination, the freedom from self consciousness, the exuberance and the rib cracking laughter of the games played when I was a kid.

But I find it can all be simulated with beer.

That.