Could someone please elaborate this sentence's meaning?
I was just surfing through internet, there I came across it.
Its advice given to people who want to begin writing (usually fiction), to write about topics and subjects that they know or have experienced. If you "write what you know", the small details will fall into place more easily, and the writing will seem more authentic.
For example, if you have never left the desert, and try to write a novel about ocean travel, you will probably make many mistakes which your readers will notice. But if you write about the desert, your personal experience will make your writing better.