Same Kanji, Different Word

Sometimes, in Japanese, a single kanji may be used to write multiple, different words.

For example, 金 is the kanji for the word kane 金, "money," and kin 金, "gold." In this case it'd be called an homograph, since both words are written exactly the same. Note that, when multiple words are written with the same kanji, each word becomes one of the kanji's readings.

Another example: hosoi 細い, "thin," and komakai 細かい, "fine," "detailed." This isn't an homograph, since the hiragana at the end of the word changes. This kana, by the way, gets called okurigana, and is used precisely to let you tell apart multiple words written with the same kanji.

A more complex example: gaikoku 外国, "outside country," a "foreign country," and kokugai 国外, "country's outside," that is, "outside of the country." Here, we have different words written with the same, multiple kanji. But in one word the kanji order is reversed, and changing the order of the kanji changes the meaning of the word.

Unrelated to the above, in Japanese a kanji may represent a morpheme, which is combined with other kanji morphemes to form whole words. For example: ko 子, "child," when combined with "man," otoko 男, forms the word "boy," danshi 男子, but with "woman," onna 女, for the word "girl," joshi 女子.

Knowing to understand a kanji's meaning this way may help you guess the meaning of words from their kanji, which might be useful.