It means: "It's another matter. / Not to talk about that here."
With more detail, it says: "That incident is not associated with this matter, (so we'd better not mention it now.)"
"둘이 싸우고 화해 안 했는데, 이번에 공동으로 작업하는 거야?" (You guys have fought and not settled yet, right? Then how are you co-working with him this time?)
"그 일은 그 일이고 이 일은 이 일이다. 일하고 개인사는 구분해야지." (It's another matter. Let us not mix business with pleasure.)
(A bit diffrent from "Let bygones be bygones.")