。
Composure is key.
Literal
Calmness [subject-が] essential is.
落ち着き is the masu-stem nominalization of 落ち着く — 'the quality of calm/composure.' 肝心 is a na-adjective meaning 'essential / crucial / vital.' The whole sentence is a pithy aphorism; 肝心 comes from 肝 (liver) + 心 (heart), both vital organs.