A confidential remark meant only for the people present or the person being told; used to preface a secret or private comment.
ここだけの話だけど、来月会社を辞めるつもりなんだ。
Just between you and me, I'm planning to quit the company next month.
ここだけの話、彼は入社試験でカンニングをしていたらしい。
This is confidential, but apparently he cheated on the job entrance exam.
ここだけの話だから、他の人には言わないでね。
This is just between us, so please don't tell anyone else.