I mean that to write competently, you have to know what you're writing about. Whether you're composing fiction or fact, song lyrics or an instruction manual, you can succeed only to the extent that you comprehend your subject. Writing a book or essay is like locking yourself in a room with only two exit doors—one door marked "Learn!" and the other "Fail!"
Of course, you don't just learn: Were that the case, every straight-A student would be a world-class writer. (Timothy Ferris at the National Writing Project website on why he writes)