Furigana (振り仮名) is a Japanese reading aid, consisting of smaller kana, or syllabic characters, printed next to a kanji (ideographic character) or other character to indicate its pronunciation. It is one type of ruby text. Furigana is also known as yomigana (読み仮名) or rubi (ルビ) in Japanese. In modern Japanese, it is mostly used to gloss rare kanji, to clarify rare, nonstandard or ambiguous kanji readings, or in children's or learners' materials. Before the post-World War II script reforms, it was more widespread. Furigana is most often written in hiragana, though katakana, alphabet letters or other kanji can also be used in certain special cases. In vertical text, tategaki, the furigana is placed to the right of the line of text; in horizontal text, yokogaki, it is placed above the line of text.
So many great and interesting uses for this than one might have thought. I particularly like the pdeudo-parenthetical way this is sometimes used. I kind of wish that Western languages had versions of this.