Of course, most of my blog topics are about things that I do know something about.
In fiction, write what you know means (to me) take a real situation and "what if" it until you have a story. Nobody wants to hear about little Suzy going to the corner store for ice cream. But if little Suzy goes to the store for ice cream and walks in on a crime or a giant cockroach convention --that might just catch our interest. Reality plus embellishment equals fiction.
Research on your topic equals knowing also. I wrote a piece where two children heard a noise and went to investigate a few hours later. They found the dead body of a man who'd terrorized them on the floor, with flies buzzing around. A reader pointed out that flies wouldn't have gathered that quickly indoors, with no open windows, etc. She had researched it. Write what you know.