The expletive or pleonastic "it" used because a noun or pronoun is required syntactically but which itself has no explicit meaning or reference.
The pronoun it can serve as an expletive pronoun or pleonastic pronoun, in this role sometimes called the dummy “it.”
Unlike a referential use where it would refer to an object, idea, or other thing, the dummy it has no meaning in and of itself, and indeed, cannot have a meaning; for example, it is not possible to replace it in either the following question or response with another noun or noun phrase:
What time is it?
It is five o'clock.
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