An emphatic particle used after a noun or phrase to stress that item (meaning roughly 'it is precisely X' or 'X is what matters').
君の努力こそ、成功の一番の理由だ。
It is your effort above all that is the main reason for success.
健康こそ、富よりも大切だと彼は言った。
He said that health, above wealth, is most important.
今の問題は時間こそが解決の鍵だ。
For the current problem, it is time that is the key to resolution.