A four-character idiom meaning extremely active or capable in many fields; versatile or a jack-of-all-trades (literally 'eight faces, six arms').
彼は研究も執筆も講演もこなす、まさに八面六臂の活躍だ。
He handles research, writing, and lectures—truly active and versatile in many fields.
イベントの準備では彼女が八面六臂の働きをしてくれたおかげで助かった。
She was instrumental in preparing the event, working tirelessly in many roles, which really helped us.
八面六臂はやや古風な表現なので、会話では『多才だ』などと表現することが多い。
八面六臂 is a somewhat old-fashioned expression, so in conversation people often say 'talented' instead.