it是形式主语，指代后置的主语从句。如果用正常语序that is said，其实就是 “one says＋that引导的宾语从句” 的被动语态，未免主语部分太长，显得结构臃肿，所以就改用 it is said that句式。
it is said that用法
it is said that是据说，据传言的意思
过去式，it was said that + 句子，【谓语用过去时】
现在时，it is said that +句子【谓语用现在时】
将来时，it will be said that +句子【谓语用将来时】
例句：
Someone says that she( works) as an actress.
It is said that she works as an actress.
She is said to work as an actress.