Its about a homeless teenager. In the music video for "Nobody's Home", Avril plays a homeless teenager who lives street-to-street with another girl. Lavigne, who said it was really fun to make, had to wear a black wig, and acid-wash jeans for the video. The other side Lavigne had to play was a rich-like version of her: in a dress, with long, curled blonde hair, singing the song with an orchestra behind her. "Nobody's Home" showed the outcasted, homeless Lavigne trying to get off the streets. She also tries to call her mother, but hangs up. In other parts of the video the outcasted girl is shown washing herself in a convenience store bathroom. She also tries playing the guitar on the street, probably trying to earn money. The video also shows the runaway trying to get inside an unlocked car during heavy rain. At the end of the video, the girl, dirty clothes, messy short hair and all, is shown with a tear rolling down her cheek. After that, she turns around and walks away. Found this on wiki.

Its a very pretty song. I believe the meaning is explained by the scene in kingdom hearts where it is played. (Or the english version of this song which is played in the English version.) You would know this song from that scene if you owned Kingdom hearts Final mix or the Japanese version. The chorus of the instrumental version of this song (in all versions) is played when sora stabs himself. One line (translated to english) "The light known as 'you' finds me in the middle of the night" describes what sora had said when no matter how far apart they are they will be always be together. Probably the rest of that part describes it too. That's why I think the meaning is described in Kingdom hearts.