Essential; crucial; of primary importance. Often used in phrases like 'what matters is...'.
肝心なのは結果ではなく努力の過程だ。
What matters is not the result but the process of effort.
説明は簡単でいい。肝心なのは実行することだ。
The explanation can be simple. The crucial thing is to put it into practice.
肝心の書類が見つからなくて困っている。
I'm in trouble because the crucial document can't be found.