The radical of a kanji character — a component used to classify and look up kanji in dictionaries.
この漢字の部首は「木」です。
The radical of this kanji is '木' (tree).
辞書で部首索引を使って漢字を調べた。
I looked up the kanji using the dictionary's radical index.
子どもに部首を覚えさせると、漢字学習が楽になります。
If you teach children the radicals, learning kanji becomes easier.