things to do; one's duty; what must be done
A set phrase meaning tasks or obligations one should carry out. Often used in contexts of responsibility, planning, or moral duty.
I listed the things I need to do today.
He quietly carried out what he had to do.
Kanji spelling using 成す; less common but still seen.
Variant with 事 for こと; less common.
A phrase composed of the classical verb 成す (なす, 'to accomplish') + the classical auxiliary べき (indicating obligation) + こと ('thing'). The combination is a fixed expression meaning 'things that should be done'.