Amirreza, if you translated this into your own language, I'm sure you would also use the word for "what." I used Google Translate here and I don't know Farsi, but this is what this quote means: اگر بیشتر مردم *آنچه* را که فکر می کنند می گفتند، حتی یک کلمه هم نمی گفتند.

We could re-write it as follows: If most people said out loud the thoughts that are going through their mind, they would be speechless. In other most people don't have any thoughts going through their mind, according to the quote anyway. "What" refers to the thoughts going through their mind or the things they are thinking or their thoughts. Beyond this, I can't help you. But maybe someone with a better understanding of English grammar can.