The principle behind it is to write those things you are familiar with.
This can be interpreted a number of different ways.
It can be interpreted as writing only those things you have direct experience of, as in travel and memoir pieces, or only writing about places you’ve actually been. This is probably the narrowest definition, and limits us most.
It can be interpreted as only writing about those experiences you can directly relate to, as in fiction set in the present day, about circumstances with which you are at least tangentially familiar. This is the mid-range definition, and one of the more commonly used ones. It is the basis upon which I’m going to take the pain radiating up from my jaw like being repeatedly sucker-punched by my own teeth and go write alien body-horror dentistry (I went to the dentist today. It is good for me, and my general health, and makes me feel like a Responsible Adult. I don’t care: it hurts now that the anesthetic has worn off, and I’m unhappy about it).
It can also be taken as an exhortation to go forth and do the fucking research. This is probably the best way to interpret it, as we should all do more research.
I had more plans for this post, talking about research resources and reading memoirs and not just statistics when researching groups, but all of you know how to use Google, and my face hurts.