1that; it — a demonstrative pronoun for something near the listener, something connected with them, or something already mentioned.
それは何ですか。
What is that?
それ、少し貸してもらえますか。
Could I borrow that for a moment?
それを聞いて、みんな安心した。
Hearing that, everyone felt relieved.
それが私の一番知りたかったことです。
That's what I most wanted to know.
2then; at that point; in that case — a temporal or situational use, often seen in expressions such as それでは and それから.
それでは、次の質問に移ります。
Well then, let's move on to the next question.
それから三年後、彼は日本に戻った。
Three years after that, he returned to Japan.
それなら、私も一緒に行きます。
In that case, I'll go with you too.
3An archaic use meaning there, referring to a place near the listener; modern Japanese normally uses そこ.
4An archaic second-person pronoun meaning you; it is not used in modern Japanese.