When you chime in, you offer your opinion or add your voice to a conversation. If your friends are discussing where to go for dinner, you could chime in, "Anywhere but Olive Garden!"
Some students are quick to chime in during classes, answering the teacher's questions and asking their own as well. When you attend a lecture by a famous writer, it's appropriate to chime in during question and answer sessions, but it's not okay to chime in while she's reading aloud from her latest book. Chime in originally meant "to join harmoniously in music."