後輩 (kōhai) means 'junior.' It is the opposite of 先輩 and refers to someone who entered a school or workplace after you, or has less experience. For example, '後輩の面倒を見る' (To look after juniors).

学校や職場で自分より後に入った人です。先輩の反対です。例：「後輩を助ける」