The film is about a young teenager who has a school project to think of something that would "make a difference" in the world. So he thinks of the idea of doing a few really major good deeds for people he doesn't even know, but telling them that in order to repay him they should "pay it forward", that is, do a few really major good deeds themselves. He is mocked in class, but what no one knows is that the idea is already catching on.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)