An idiom meaning 'it goes without saying' or 'needless to say' — something that doesn't need to be stated.
彼の才能は言わずもがな、誰もが認めている。
His talent goes without saying; everyone acknowledges it.
安全第一は言わずもがなだが、確認は怠らないでください。
Safety comes first, needless to say, but please don't neglect checks.
言わずもがな、期限は守ってください。
Needless to say, please meet the deadline.
That would better be left unsaid; something one ought not to mention — used to indicate it's preferable not to speak of it.
詳細は言わずもがなだが、状況はかなり深刻だ。
It's better not to say the details, but the situation is quite serious.
彼の失敗については言わずもがな、当人が一番辛いだろう。
It's better left unsaid about his failure; he himself probably feels it most painfully.