The impersonal voice is a style of English used in formal situations such as official letters and notices. It is used for passive sentences where there is no subject.
The window was broken. (personal - the window is the subject)
It is said that ghosts don't exist. (impersonal - there is no subject)
Sentences using the impersonal voice often begin with the phrases below.
It is said that...
It is thought that...
It is believed that...