Indirect characterization is characterization that occurs when the author does NOT explicitly tell the reader about a personality trait. If I told you that my brother is patient and kind, that would be direct characterization. If I said that he spent all day teaching his son to tie his shoes, that would tell you about patience and kindness and perhaps a few other things.
There are a few tools that authors use to indirectly characterize someone. That character's speech, their thoughts, and their actions can indicate much about their character. The character's effects on other characters is also good to notice.
One example of indirect characterization from "Stargirl" is her playing the ukulele in the cafeteria. That's not exactly something that most students do in real life or the book. It tells the reader that she is different and unique. It tells the reader that she doesn't care so much about social norms. It tells the reader that she is confident with herself.
Other actions of Stargirl herself tell the reader about her personality. Her cheering for the other team and comforting players from the other team are misinterpreted by characters in the book, but the reader learns a lot about her character in general. She wants people to do well and be well no matter who they are. Time and time again the reader sees her empathizing with all kinds of different characters going through a range of emotions. If it was direct characterization, the author would have told the reader flat out "she is empathetic and shares emotions with others well." That's not what happened though. The authors shows that.