We already know that Japanese have more than one type of writing. Hiragana, katakana and kanji. And no, furigana is not the 4th type of writing that you need to learn, so all is good!
But what is it then?
Basically Furigana is an information, written in hiragana/katakana about the pronunciation of a word.
There are thousand of kanji character in Japanese. And not all people are able to read them. They might know the word though. Not to mention that one kanji can be read with many different ways.
Based on the reader target, publisher will add proper furigana to a kanji character to avoid the reader mispronounce the word. Most of the time it will be used for uncommon kanji or name. For this purpose, the furigana will be written with hiragana. For example, 勉強 will have べんきょう (benkyou) as furigana.
Sometimes, foreign words will also get a furigana, written with katakana to inform the reader about the pronunciation. Check out this example below.
Furigana is especially helpful when you start learning Japanese! It will make you able to look the word in the dictionary much easier (else, you will need to analyze the radical). Most of children book will have simple kanji and furigana. And that’s the kind of book you need to have at first.
By the time you’re able to read the kanji, you need to get a book with much less furigana to practice your reading. Most of material (game/manga/book) for elementary level or higher will be good.
The higher the level of material, the furigana will only used on very uncommon words.
Hope it helps! Happy learning!~