Furigana is hiragana printed beside each kanji on Japanese books. For example, 「私(わたし)は学校(がっこう)で英語(えいご)を勉強(べんきょう)しています。(Watashi ha gakkou de eigo wo benkyou shite imasu.)」. In this case, "(わたし)", "(がっこう)", "(えいご)" and "(べんきょう)" are furigana.

Furigana is something you put Hiragana or Katakana, usually on the top of the kanjis. It's like a caption that guide for reading them. It is used to accurate to read the Kanjis certain ways for documents such as your ID as well. You may concern that Kanji can be pronounced more than one way. And of course, you need these captions which is "Furigana" for Kanji when you are learning Kanji as well.

Furigana is also known as yomigana

They are the hiragana endings on kanji stems.