Truth in fiction is an interesting idea. It sounds like an oxymoron but it's not. A story needs its own truth: a set of facts that are true. They don't need to confirm to the physical rules of our universe but they need to be consistent by themselves. For example, rules for magic, rules for jump drives, that sort of thing. You also need a consistent history that makes sense.
Once upon a time, I made the rookie mistake of not defining my history before I wrote my story only to find it changing out from under me as I edited. Later I developed a backstory and stuck to it, refining as I went but not modifying it greatly.
How detailed does all this information need to be? Enough so you can answer any question about that might come up. For example, when casting a spell, what happens? Does the spellcaster lose a body part or just gets tired? In terms of a backstory, you should know the major characters, their goals, ambitions, and so on. In general you will need far more backstory than you think. Most of it will be off-camera but it is important because it drives things that are on camera.
Example: In my fantasy novel, the main character fights necromancers. This requires me to develop rules around necromancers. How are they organized? What are their goals? Why is it that never seem to go away? One group is captured or killed and two more spring up. What's that about?
How I answer these questions determines much about the story in which they appear and the reaction and approach the main character takes in combatting them. For example, in my first novel, they seek to steal a magic gem to create a mind control device. But in the second novel, they abduct the local nobility on a hunting trip and have him possessed. Their hope in that case is to have the demon possessing the noble rule the land.
But I chose the title for this post for another reason too. The fiction you want must reveal the truth as well as have it's own truth. As I mentioned in another post, I have a scene with two lovers based loosely on my relationship with my wife. The scene shows the man's reaction much the way I would react, and I think, much the way most men would react, to this woman. In that scene it is universal and most people can identify with because it reveals a truth about relationships.
That's said, I don't often find places to include such scenes. But they do present themselves. Watch for them and take advantage of them. The reader will appreciate it I think.