头儿 is a colloquial noun that extends the meaning of 头 (head). In everyday speech, it often means 'boss' or 'leader' — the person in charge. It can also refer to the end or tip of something physical (e.g., the end of a thread) or the beginning of something (e.g., 'from the start'). In Chinese-character study, it can refer to the radical (部首), but that use is more technical.