The most crucial or essential point; something of primary importance that must not be overlooked.
議論の肝心要を押さえてから結論を出そう。
Let's pin down the crucial points of the discussion before drawing a conclusion.
計画は細かい部分も大切だが、肝心要は目的を明確にすることだ。
Details are important in a plan, but the essential thing is to clarify the objective.
肝心要の部分を失敗すると全体が台無しになる。
If you fail at the crucial part, the whole thing will be ruined.