Note: This post definitely needs to be prefaced with "As I understand it . . . "
;-)
In our own language system, letters have no individual meaning. It's only when they're put together to form words that they take on meaning. Hirigana is (very roughly) similar in that each of the characters represents a spoken syllable and it's only when those syllables are put together that word are formed. There's also an additional set of characters (again representing syllables, rather than words), forming katakana, which is used for foreign and scientific terms. The two together form 'kana'.
However Japanese has also absorbed Chinese symbols into the language, where each symbol represents a different word (or a number of different words, with the appropriate one being determined by the context). That system is 'kanji'.
Written Japanese mixes both systems.
Kawaii is usually written in hirigana but technology may substitute the kanji version.