The adage "write what you know" has brought up a lot of discussions over the years and has been open to interpretation by any and all. Here's mine:
In simple terms, write about life as you honestly see it or have lived it. Take your experiences, your flashes of comprehension about the human condition, things you've witnessed or heard of from others and use them to better your writing. It's the things we know and come to know about life, about relationships, about what it is to be human that enriches what we write.
For an example, I took a couple of school experiences I had that were negative and embarrassing and meshed them together, added a spin that fit the character and story I needed them for, created new people in place of the original ones, and wrote two of the most powerful chapters in my WIP. Truth seeps from the dialogue, the injustices are felt more keenly, and I don't have quivers of self-doubt regarding them. I wrote what I knew and my beta readers have responded well to those chapters.
You never know what common or uncommon daily occurrences can give you moments of truth for your story. I took two incidents with mice, one from the past, one from the not-so-past and again fused them to use as the leverage that gets my two main characters in motion during one part of the story. I didn't have to make up any old plot ploy to incite the action, I wrote what I knew and it did the job.
The same thing goes for social interaction, internal thoughts, or any type of understanding regarding culture, mythology, or even background. Unless we're cloistered up in a small room with our meals slid in under the door, we have tons of material to work with when writing. The great part is we all have unique points of view. Two people may have a similar experience but respond or learn something differently from each other. The social tiers we work, live, and play in also give us fodder for inserting truth into our writing.
A slight caution: don't write your character truths exactly as real life people undisguised, especially in fiction. Writing about specific people is dangerous and you could run into huge problems if you try to pull a stunt like that. Pull bits and pieces of someone else's character if you need to but change up how they look, what they do, or even who they are. The important part is the truth you experienced in the interchange with that person, what you learned, or what happened that you can play with in a fictional story. Demonizing someone or a group of people on purpose and undisguised is unethical.
There’s also the obvious truths to find in what we do for a living or have educational experience in. This part often stymies writers. Sometimes we think “Well, I never was an acrobat so how can I write from that POV?” Remember, we’re all still living and learning and it’s never too late to learn something new. Sure, you might not go out and take gymnastics or roam with a circus for credibility but you can do research, talk to people who do have acrobatic experience, and do a good job writing from that POV anyway. The truths of your story will come out in other areas. Maybe your acrobat character has a bad past with one of their parents and that reflects something you’ve experienced first-hand. Or maybe the acrobat is color blind, just like your best friend growing up.
The overall theme (hidden or otherwise) of your story may deeply reflect what you know. Perhaps your main character’s inner journey deals with rejection and finding personal acceptance—something you may very well have struggled with or have known someone else to struggle with. Maybe that yearning which propels the protagonist on their quest reflects your own needs to right wrongs, find justice or peace, or loosely follows a road trip you took when you left home.
Discovering and using what you know isn’t hard. Truths are easy to find.
Tell me about how you've been able to insert truth into your writing, or a time when you've read a story and felt the writer really knew what they were doing when they wrote a certain character, scene, or conflict.