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 with copulative verbs  in order to produce a grammatical sentence. Some examples :

It's raining
It's hot.
It's time to go.
It's already lunchtime.
It's late.
It's a long way to Tipperary.
It wasn't very nice in the hotel.
It seems that he left.
It looks as if we'll be late.

It makes no sense to ask What is raining? What is late? What's a long way to Tipperary? What seems that he left? 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.

There can also be used as a dummy subject for copulative verbs to talk about something existing or happening, or involving numbers  :

There's a message for you
There seems to be a problem.
There was an eclipse yesterday.
There were thousands of people at the demonstration