Please look at the diagram below.
If you click it, an enlarged view will appear.
Your brother drew a simple diagram of the relationship between him and his girlfriend.
The kanji and the word are exactly the same. That means they share meanings as well.
While words that are made up of a single kanji usually take the kun'yomi reading, this one takes the on'yomi reading. Luckily, this is the same reading you learned with the kanji, so it shouldn't be too hard to remember.