When I first heard this bit of advice I was puzzled. I thought this would tie me into writing just about the career I was part of. I was a nurse and yes, I began my career writing nurse romances but then I decided to expand my horizons. I wasn't sure that bit of advice would allow me to write mysteries or fantasy, two of the areas I enjoyed reading. So I sat down to think about what it really meant.
What sort of things did I know? One area was that of emotions. I knew about love and hate and myriad other emotions I'd experienced. So the emotions became a way to explore the other areas of writing I wanted to explore.
Because I had been a nurse I understood how other people reacted to news good and bad. I was also a people watcher. It's not that I knew everything about people, their actions and interactions but I knew wnough to expand on these areas.
I had lived in various places and these became part of what I was able to write about. New Your, Pennsylvania, Ohio, Oklahoma, Texas. Small towns, cities, rural areas. I learned I could transplant these to the area where I wanted to write about.
Another place where I knew things was in the use of the senses. Visual effects, tastes, smells, sounds and touch. I could use things I remembered to add to my stories.
So, write what you know but first sit down and think about all the things you know. Write about the things you can change about what you know by expanding or subtracting.