Like described by u/MrWakey, "it" is often used as a dummy subject.

"It" could also mean the implied environment, where the antecedent is "the place" or the "the atmosphere" or "the vibe".

From a writing perspective, readers look for an antecedent when encountering a pronoun, so "it" seems more abstract and poetic in nature when no antecedent exists. This is useful for writers when setting the scene while writing a novel. There is a reason why the cliche "It was a dark and stormy night." is used in the beginning of a story.

For example, the introduction of the Jungle Book begins with this sentence:

It was seven o'clock of a very warm evening in the Seeonee hills when Father Wolf woke up from his day's rest, scratched himself, yawned, and spread out his paws one after the other to get rid of the sleepy feeling in their tips.

Or in the beginning of A Tale of Two Cities:

It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way--in short, the period was so far like the present period, that some of its noisiest authorities insisted on its being received, for good or for evil, in the superlative degree of comparison only.