In written Japanese, these are small hiragana written above a kanji to indicate its reading. Furigana are present in writing intended for children, such as storybooks and periodicals, and often in Japanese language textbooks.

It has become relatively common in manga to use furigana that actually don't correspond to the kanji they're used with, to do certain kinds of word plays. For example, a character might say "Listen to the expert!", but the furigana for "expert" actually read "fool", to indicate that the remark was meant sarcastic.