more importantly; rather than that; but
Used to shift the topic or introduce a more important point, often contrasting with what was just said. Can be translated as 'more importantly', 'rather than that', or simply 'but' depending on context.
Before that, finish your homework first.
More importantly, have you prepared for tomorrow's meeting?
旅行の話もいいけど、それより仕事を終わらせよう。
Talking about the trip is fine, but let's finish our work first.
それより is a casual way to pivot to a new, often unrelated topic ('more importantly...'), while それはさておき more pointedly sets aside the previous remark to return to the original subject.
Compound of それ (that) and より (than), literally 'than that'. The conjunction use developed from the comparative structure.