Hiragana is one of four writing systems used in Japanese (the others being katakana, kanji, and rōmaji). It is the most elemental of the four, and is the first one taught to children.
Think of hiragana as the Japanese alphabet. Any Japanese sentence (that doesn't contain certain foreign loan words) can be written entirely in hiragana, since 100% of the sounds used in Japanese have a matching hiragana character.
Each hiragana character is pronounced only one way, and with few exceptions, there are no two characters with the same pronunciation.
Exceptions include お and を (pronounced ‘o’), じ and ぢ ('ji'), and ず and づ ('zu'). Historically, they sounded different, but they are indistinguishable in modern Japanese.