This is a great phrase from the song ‘Ours’ by Taylor Swift. It is very descriptive of the people who are bullied, an it fits the idea of bullying:
“People throw rocks at things that shine.”
This song expertly says that the people that stand out in the crowd are the ones that bullies will pick on. Because most people who stand out have a very positive self-confidence, the bullies get the idea to steal that self-confidence for themselves.