What does "it" mean which is at the top of the sentence?
The first "it" in this sentence is used as an anticipatory verb subject or an empty subject before a verb. "It" is used to anticipate the subject which comes later in the sentence, and is used when the subject order is different from the true subject coming first in the sentence. Here's a few examples of "it" used in similar contexts: Ex: It was nice to visit them again. Ex: It is good that you did well on your test.