Part
Portion, Content
noun, prefix, suffix
This word has the same meaning that you learned with the kanji: part, or portion. By extension, it can also mean content, as in "alcohol content" because that's the portion of alcohol in a drink.
This word is used for fractions, too, as in one third or one quarter. Again, you can see how this links to "parts" and "portions", right?
ぶん
This word is usually combined with other words, which is probably why this single kanji vocab word uses the on'yomi instead of the kun'yomi. If you know the kanji reading, you know this vocab's reading as well, but the reading is different depending on the meaning:
If it means part or portion, it's read ぶん. Probably because your buns come in two parts. Ohoho.
その玉ねぎは女王の分だ。
That onion is the queen's portion.
このウィスキーのアルコール分は四十パーセントです。
The alcohol content of this whisky is forty percent.
この市はシアトル市の3分の1の大きさです。
This city is one-third the size of Seattle.