It’s a gate. It’s a place to start, whether you’re describing a character or a place or an action. And you know more than you think.
Especially if you write science fiction or fantasy. Because then you’re free to make up the things you know from whole cloth in a few spots, maybe many. That’s a privilege other genres don’t have like we get in SFFPHM (Science Fiction, Fantasy, Paranormal, Horror, Magical realism). Other writers might make up a town or something, but their worlds are expected to act totally like the real world.
Settings aside, writing what you know also doesn’t mean every character has to be a writer or a retail person or a middle manager or whatever variety of things it might be that you do with your lifetime. Doesn’t mean all your aliens need to know what it’s like to poop or vomit. Doesn’t mean that your orcs need to be the orcs you know from all those movies with the really short dudes and a jewelry fetish.
Write what you know is great for writing the human bit of your story, though it’s a damn good thing to remember that if you’re writing about a culture or subculture you’re not familiar with, it’s best to consult members of that culture. Unless it’s elves. You’re not going to find any real elves to consult about your fictional ones, I’m afraid. HOWEVER that said if you’re going to base your elves’ culture strongly on the culture of southern Spain, you’d better be familiar with southern Spain in some significant way.
’nuff said. I’ve already harped on that enough and I hope you were listening.
Write what you know means the practical simple things around you, sure. Do use your memories, your pain, your joy, things that happened to you, places and people you’ve met (though tread lightly when writing a person who’s close to you lest ye complicate yer relationship).
Use the things that made you grieve. Use the things that made you weep with joy. You can choose not to reveal that those things came from within yourself if that’s what you want or need to do. Do use your writerly skills to file the serial numbers off them.
Write things you know you can feel. Because feeling (Hemingway would say, did say, bleeding) on the page speaks to readers. It can reach into your readers’ hearts where mere skill can only titillate their minds.
There are plenty of successful authors who do the latter. But the stories that do the former are remembered. They change minds and sometimes lives.
Don’t think it’s easy, though. No author succeeds at that every time. In fact, I think it’s an ideal that is rarely reached.
Do reach for it.