Write what you know is, I'm pretty sure, the most commonly given advice to new writers. Even not so new writers. I've been hearing it for years now. And it seems so stupidly simple that there should be no explanation needed, right? You should write about the things you know and understand. Easy. But maybe not.
At least, I hope not. If we all just wrote what we knew, can you imagine the stories we wouldn't have? I've never actually encountered a zombie, Jack hasn't been hired to drive anyone to suicide, CJ's never been dead. Kaz hasn't met any faeries, Ben's never killed anyone, and Shannon's never been a teacher. At least, not that I know of. But we've all got, in my opinion, some pretty good stories about those things.
I think it might be better if people said, "Be inspired by what you know". I think that's closer to the truth anyway.
I was in workshop when someone asked (the entire class, not just me), "Do you think it's harder to write fiction or non-fiction?" We were talking about the truth at the time and how writing non-fiction meant admitting a lot of things that were maybe not so easy to admit. I said then, and I still believe, that when to take into account the confessional nature of writing, they're equally difficult. Even the most fantastical piece of fiction is littered with truths. Some of them are beautiful, some are horrible, but they're all rooted in truth.
So, please, when you're inevitably told to write what you know, don't let that limit you. Don't only write about female waitresses or shop girls. Don't only write about broody writers or middle-aged men looking for themselves. Don't only write about today. Don't only write about the place you're from or where you live. Write about them all, write about everything. Be inspired by what's in your head and let that knowledge make your worlds real.