The reality of your setting is what the players actually experience, not what you wrote in the setting document.

I'm gonna expand on this, because I think it's an interesting thing to consider, and an important lesson for players and DMs alike.

If you, as a player, write a big complex backstory full of important and interesting events for your character, but keep it hidden from the other players - that backstory essentially doesn't exist to the table. Yes it can affect how you think about your character, but it's not a part of the collective story until it impacts the table. This can have negative outcomes in roleplay.

If you, as a DM, write a bunch of secret information for your NPCs, but the players never see it, it's essentially not real to them. If the knights of your city have a super cool wyvern emergency response team, but the players never see it, that detail never existed.

Let's say, for a random example, that you grew up as an orphan in a dwarf mining colony, and your parents were... not the best. you experienced abuse and discrimination, both from them and the dwarves in the colony. Eventually fled and made your own life elsewhere. You've decided to write on your sheet that your character hates dwarven culture, and mistrusts dwarves, and always views their actions in the most negative light possible. Now let's say this childhood trauma is so bad you don't want to think about it, or talk about it with your party members. You keep it secret.

Now, let's say the DM wants to bring this part of your story into the overall story, so they set you on an adventure that involves diplomacy with a local dwarven mine, or they give you a dwarven NPC to travel with the party, or some other dwarf-centric plot. If your character acts "weird" around the dwarves, constantly refusing to trust them, or speaking ill of them, or looking for malfeasance where there is none - in your head you're just playing to character, and your actions make sense. The other players don't know your history, so what they see is completely arbitrary prejudice.

To those players, your backstory isn't a part of the story they've experienced or the world until you bring it to the table. Your actions and decisions might not make sense to them, or seem out of character. Your choices might be incredibly frustrating to the rest of the table when they obstruct or interfere what they perceive to be the party goal. Without the context of why your character is like that, their experience of your character is massively different to your own.

Here's a second example: Your character used to live somewhere far away, they committed a murder, then fled their city, changed their name, and came to (wherever the campaign is set) to start over. They're ashamed of what they did, and don't want anyone finding out for fear of being tracked down and brought to justice, so they'll never tell anyone about it.

Now let's say your DM is running... Dungeon of the Mad Mage... a mega dungeon plot where the characters go into the dungeon, then fight their way through monsters until they're level 20, and never see civilization. They never have an opportunity to bring your secret to the table and make it part of the story. Or let's say they're just busy with other plots and forget.

This backstory detail might be important to you - but the other players never see it. From their perspective it never happened at all, it wasn't part of the narrative, it's not an extra dimension to your character, and it's not an event that happened in the world - they just don't know about it... so it's not real from their perspective.

Now, I'm not advocating against characters having secrets, or DMs having intrigue in their plot that drives outcomes without the players seeing it directly. If you want depth in your storytelling, it's important to have flaws and phobias and secrets, and opportunities for your character to grow and all that good stuff.

What I'm saying is, if you have an important secret that you haven't told the other characters, it can be worth thinking about what your character is doing from their perspective. What does it look like to a person who doesn't know your secret? Where are the differences between the story you're telling yourself, and the story you're telling them?

When designing "hidden content" consider what circumstances might cause your character to reveal their secret. If your intention is to keep it hidden for the whole campaign, then think about what that does to the collective story.

This lesson is especially important for DMs, because it's so easy to devote hours and hours of planning to things the players might not discover. It's often important in your story design to have things going on that the players don't know about, so they can unearth them. Mysteries need secrets... but when planning your campaigns, always consider the questions "when do the players learn this?" and "how do the players learn this?" because until they do, it's not a part of their world,