Furigana characters are printed on top of kanji characters, read more about them here. They are a common tool for making Japanese easier to read, because kanji can be read differently, depending on the context. A common question beginners often ask is: How should I learn all these the kanji readings? In short, it’s not recommended to learn these readings individually. A long answer to this is written down in Tae Kim’s blog.
For those of you who are not yet familiar with how the Japanese writing systems work, have a look at this video .