If you're a writer, I very much doubt your life experiences are actually very interesting. I take this from my own life (or, "what I know"), so your mileage may vary -- but even if you've led a fascinating life full of adventure and intrigue, those experiences are probably good for one novel at most. If you're planning to just write the one book, good on you -- you now have my permission to write what you know.
The rest of us have to make shit up. It's quite a bit more fun, and the stories are generally better than thinly disguised retellings of our own mundane existences. I think the phrase should be rearranged to read "know what you write," as that's a more helpful piece of advice.
Come up with your story idea, then research the hell out of it. Want to write murder mystery with a cop as the main character? Talk to some cops and get a good idea of how their world works. Want to pen a novel about a girl's cheerleading team (if so, what the hell are you doing here)? Go hang with the cheerleaders, but be careful to avoid arrest on charges of corrupting a minor, stalking, or anything similar. Perhaps talk to former cheerleaders -- that'd be safer.
In this day and age, you can get a good feel for almost anything by using the good ol' Internet (of course, this assumes you're bright enough to write a book in the first place -- there's a lot of crap out there, and you'll have to separate the good info from the obvious bullshit). So write whatever you want -- but make sure to do your homework and know what you're talking about.
And now, a picture of the Tampa Scientology Center, for no reason other than it was on my hard drive. I think I took it when I was wandering around downtown Tampa looking for illegal cigars.

(I never said it would be an interesting picture.)

