That was at first, I started slowly understanding what he meant. We were first time writers (in a way), so we didn't know how to build a world from nothing. We had to start with something, it was an easy way to start. Yet, today I feel like this could hold you back in a way.
Yes, do write what you know, BUT what I do is if I haven't been to a place, I look it up. I make up a new world within this world. If I don't know a lot about something, I research it too the end of the world.
For example, I am working on a story about Sidhe; which is the fairy people of Irish folklore, said to live beneath the hills and often identified as the remnant of the ancient Tuatha De Danann. Like him:
Like I can't find a lot on them, I have to read as much as book about Fairies and anything that could do with Sidhe. Today, my awesome boyfriend buy me Thrones of Glass by Sarah J. Maas, and I finished last week A Court of Thorns and Roses by Sarah J. Mass.
I guess what I'm trying to say is that you have to write what you know and research what you don't know. The more you research on something, the more it seem like you know it. Don't forget about the weather in a location! (which I almost forgot in my story lol)