Honestly it's weird that several of the comments in this thread seem to genuinely think OOP is talking about coloring books, just because of the joke in the title.

But yeah, furigana (also called rubi or yomigama) are basically just glosses, usually indicating how a character is supposed to be pronounced, but sometimes used for other purposes like wordplay or explaining technical terms. They were pretty widespread in Japanese before the postwar writing reforms in all sorts of media, which is somewhat amusing because that's also when furigana were most "useless" due to all the centuries of historical spelling.

In any case, nowadays furigana are most widespread in media for young people and language learners, and outside of that context are mainly used for, like, proper nouns and particularly rare words. Which isn't to say that there's no furigana'd content for L1 adults at all, just that it's rarer. You can write furigana on Lemmy too: {Glossed text|Gloss} = {Glossed text|Gloss}. Use that markdown for Kanji, use it to gloss sentences from one language to another, use it for non-manual features in sign language glosses, use it to include brief translation notes, use it for whatever.