https://aethermug.com/posts/the-beautiful-dissociation-of-the-japanese-language
The Japanese scholars-aristocrats began repurposing the Chinese characters, which they called kanji (for, well, "Chinese characters"). Sometimes, instead of using them for their meaning, they used them for (gasp!) their pronunciation. By ignoring the original content of a kanji, they could string them together to form almost any sound.
Over the centuries those "sound-only" kanji, called man'yougana, evolved into something else entirely. They became simpler, more streamlined, and more standardized. Where the symbols were originally composed of many short strokes, they gradually lost detail and complexity. Where the scribes could choose between a slew of different kanji for any given sound (for instance, the sound pa could be represented by any of 20 characters), later the number of options dwindled and eventually settled to two.
That's how the two syllable-based alphabets in use today, hiragana and katakana, came about (collectively kana).
furigana. These are tiny kana characters showing you how to pronounce a difficult kanji.
My favorite are all the versions of the word "cousins" meant in a reciprocal sense, as in "she and I are cousins (of each other)". In spoken language, you just say itoko in all cases, and that's it. In written language, you use the appropriate combinations of the kanji 兄 (ani, older brother), 弟 (otouto, younger brother), 姉 (ane, older sister), and 妹 (imouto, younger sister), preceded by the kanji 従 (shitagau, accompany), to specify the exact genders in the relationship. (These are also jukujikun, come to think of it.)
Gikun is the replacement of a kanji's or word's normal pronunciation with something else through furigana. Novelists and manga-ka use it to inject an almost subliminal layer of meaning beyond what is afforded by the words and kanji. It achieves an effect similar to a textual voice over, at the same time as the actual text you're reading.
You see it a lot in manga: the actual kanji say something, but the furigana, instead of giving you the true pronunciation of the word, give you something else entirely. Sometimes it's a synonym of the word with a more pungent nuance
The author of the excellent gikun explanaton on japanesewithanime.com clarifies the context:
Todoroki Shouto 轟焦凍 has both cold and heat abilities, which come from the sides of his body: from the right comes cold, from the left comes heat.
Here Shouto, the protagonist, is telling the flame-y man "during combat, I won't use my heat power for any reason at all." Being a kids' comic, all kanji have furigana. But the kanji for "heat" (highlighted in red) comes with an unexpected reading. Instead of the official netsu, the furigana reads hidari, which means "left".
So the reader gets two messages at the same time: the character says "I won't use the left", but the text is saying "I won't use heat".