I haven't hit many of these 'how to' bits since summer, being swept up in this project, work stuff, and recent tragedies, so I think it's time.
This phrase is shot out at baby writers as often as 'show, don't tell'; and probably receives the same number of sage nods combined with vapid expressions.
What the hell does it mean?
It's one that baby writers like to argue about - how can you write about goblins and dragons and such, if you CAN'T know them - they're imaginary.
It's about writing things that you've experienced and understand, certainly. By the time a person is 15, they understand longing, social gaffs, the pain of failure, etc. It would be a bit more difficult for someone of this age to write about the inner workings of a long term marriage from the inside, or convincingly represent an industrial engineer from a mathematical basis.
Writing isn't about facts, it's about personalities and emotions. As you grow older, you meet more personalities, and relationships become more complex.
The core of this theory is that a beginning writer will have greater difficulty writing something they are entirely unfamiliar with, both from a technical and emotional standpoint.
I've never been a police officer. Could I write one? Sure. I would need to do enough research to make it buyable, though. How long is training? (Depends upon what type of officer). How does the ranking work, what are the basic skills, social atmosphere, acceptable/typical behavior, etc. The character would know this stuff.
For someone just starting out, all that research might prove daunting, and hold up the progression of actual practice.
If someone has never been in love, experienced major losses, they are more of a challenge to evoke.
That's what it really means - taking small bites. Starting with the familiar territory as a jumping off point.
And it can be far more pleasurable for the writer to begin somewhere that is interesting to them. Love dragons and pixies? Then I bet you've read enough folklore to know the difference between a pixie and a skeltie. Love firearms? Then you've got a base for the difference between a Firestar and a musket - and enough knowledge to rough out what kind of character would use each, and how.
Or the world of competitive bodybuilding, gambling, art galleries...the list could go on forever!
As you gain experiences, your knowledge base and interests will broaden, and you'll be able to write more off-the-cuff on a variety of situations, objects, time periods, etc., and make it real.
What will always feel the most legitimate, are the things you know best. So write what you know.
And you know a lot.