The 'pay it forward' original story is about a concept where a good deed is repaid by doing a good deed for someone else instead of the original benefactor. It's a story of kindness spreading in an unconventional way. For example, if someone helps you, instead of paying them back directly, you help another person in need.
It depends on the specific version of the 'pay it forward' story, but often there is a protagonist who starts the chain of paying it forward, like a young boy or girl with a big heart.