You know what 属口语用法,通常用于讲话的开头,表示接下来的话会令人吃惊,很少人知道,或表示强调等。英语解释是:to introduce a piece of information which is surprising, which is not generally known, or which you want to emphasize.
you know what是习语,可以理解为你知道吗?
例如:
You know what? I think it's time to go home.
what应该是代词,代指后面跟的句子。