There are two roles in a DnD table: the GM/DM/Referee and the Players/Heroes/PCs.
Players ask questions about the world and tell the GM what their actions are.
The GMdescribes the world and its elements, and tells the players what the consequences of their actions are.
Rulings, not Rules
The game is flexible. There are standard procedures, but if you have a cool idea during play, it might later be integrated as a house rule.
The rules can always be bent, as long as the proposed alternative is convincing and fun.
You can always try to do anything. Succeeding is another story.
Motivation and Ethos
Emergent storytelling.
PCs are looking for treasure and adventure. Going out to the wilderness is its own reward. 1 gold = 1 xp.
There might be a narrative arch underlying the story, but in general in this game we explore a world, not character narratives. The latter will come naturally the more we play.
Polar opposites.
The game is built around the idea of opposites:
Order vs. chaos.
Civilisation vs. nature.
Human vs. monster.
Known vs. unknown.
Opposing factions.
There's no absolute good or bad. You will find instead many factions fighting for what they want.
Survival and Exploration
The game is not balanced, and it's potentially lethal.
It's tacitly assumed that the characters are being careful as they move around, and are always analysing their surroundings.
The GM will make the best effort to telegraph danger. If a trap is triggered out of nowhere and a PC dies without having had a sense of danger, that's a GMing flaw.
During encounters, try to figure out what intelligent beings want before you fight them. Combat is almost never the optimal way out of things.
Keep track of time and resources.
Your main resources are time, light (daylight, torches) and food (rations).
Exploration is three-layered. Features in the world can be either:
Overt: obvious features that you notice as soon as you reach a place.
Hidden: features/places that you find if you explore a region. Nospecificaction is necessary to unfold them, other than simply poking around.
Secret: features that require a specific set of actions to be triggered.