Hiragana 平仮名 is a syllabary, used to write:
-inflectional endings for adjectives and verbs (okurigana 送りがな);
-grammatical particles (joshi 助詞);
-Japanese words with no kanji, or where the author didn't know the kanji, or where the kanji is likely to be unknown to the intended readership;
-indications of how to read kanji (furigana ふりがな);
-some common words which are perceived as easier to read in hiragana than kanji: for example, takusan (many) is more frequently written たくさん instead of 沢山.
Hiragana is used for any Japanese origin word that does not have a kanji reading, or if the person writing does not know the correct kanji for the word. But it is mostly used for any Japanese word that is not a noun, verb, or adjective, specifically for Japanese particles, pronouns, conjunctions, interrogatives, and honorifics. Hiragana is also used for the congugated endings of verbs, adverbs, and adjectives.