This song is about how Amy was feeling when her sister, Bonnie died. When Amy was 6 years old she was told that her sister Bonnie who was 3 years old, had died. She was in the school yard when she found out.
This song shows how when you are 6 you don’t quite understand what is going on and she didn't quite believe what was happening. What she's saying in the song is what she felt when she found out, and since she was so young, she didn't fully understand the concept of death.