The demonstrative pronoun 'それ' used as a subject/topic introducer — 'that' or 'it' referring to something previously mentioned.
それが問題の本質だと思う。
I think that is the essence of the problem.
彼の言ったこと、それが真実かどうか確認したい。
I want to confirm whether what he said is true or not.
A discourse marker used to hesitate or preface an answer — similar to 'well, actually...' or 'well, the thing is...'.
「来週の件なんだけど」「それが…ちょっと都合が悪くて。」
“About next week...” “Well, actually... it's a bit inconvenient.”
それが、まだ決めかねているんです。
Well, actually, I still can't decide.