“Write what you know.”
One of the most important things, as a writer, is your ability to create a connection between your writing and your reader. Establishing a bond between your characters and your audience creates empathy in your reader, and provides them with a closer ‘experience’ of your characters’ adventures. Creating this connection with your reader is very important as this is what makes the difference between them putting your book down after only one or two pages, or reading through to the end.
Obviously, a lot of stories, genres, themes are set in times and settings that it is unlikely their authors have first-hand experience on which to dwell. I myself have writing that explores the future, the solar system, and the apocalypse, and I can say with some certainty that I have never encountered any of these things. However, I do have experience of what it is to be human, of what it is to feel emotion, of what it is to encounter a new place or new people. And, I can use these experiences to develop a more realistic and important portrayal of my characters’ own path.
Essentially, “write what you know” refers to this concept – that you can use your own experiences and feelings (et al) to better portray, or describe, circumstances encountered by your character(s). It’s a phrase best not taken literally, at face value, but in little glimpses of the human condition: you know how you feel when you feel fear, when you feel joy, or sadness, just as you know what a book page feels like, or a boot lace, or a flower petal. You know what rain sounds like, what early morning dew-covered grass smells like, or the sound of rock grazing against metal. Use your knowledge of these things to bring life to your writing – to your characters, and the settings and circumstances they encounter.