A subcommittee is a small group taken out of a larger committee and given one narrow job to do. A committee of thirty people cannot examine a budget line by line, so it appoints six of its members as a subcommittee to do that work and report back. The subcommittee usually does not have final power: it studies, drafts, and recommends, and the full committee decides.

The word belongs to formal organised life — parliaments, city councils, schools, companies, professional bodies, university departments. In the United States, congressional subcommittees are powerful and often in the news, so American political reporting uses the word constantly. In Taiwan you will meet it in business English, in international-organisation reading passages, and in TOEIC-style listening about meetings.

It is countable: a subcommittee, two subcommittees. It takes of or on for its topic — a subcommittee on housing, the finance subcommittee. Like committee, it can take a singular or plural verb in British English ("the subcommittee have agreed"), while American English keeps it singular. Written as one word, no hyphen.