An ELT Glossary : Empty "it" / Dummy Subject
Empty "it" (also known as the dummy subject) is a pronoun that has no referent and no meaning, but which is syntactically necessary in order to produce a grammatical sentence. Some examples :
It's time to go.
It's already lunchtime.
It's a long way to Tipperary.
It wasn't very nice in the hotel.
It makes no sense to ask What is raining? What is late? What's a long way to Tipperary? etc. These are verbs which have no real subject - but syntactically English demands that one is present. To "fill the gap" the dummy pronoun it is used.
For a list of the uses of the dummy pronoun it, see https://learnenglish.britishcouncil.org/en/english-grammar/pronouns/it-and-there