Pay back is if someone did something nice for you, you would then do something nice for them because of their original kindness. Pass it on means you would be nice to someone else. So today i made cookies for my friend. Then she passed them on for someone else to eat. That is a very weird way of putting it but now imagine that concept with being nice...so i helped someone whose car was broken down on the side of the road, so then they later see someone drop their bag, then they would pick the bag up for that person. In america we have an expression, pay it forward, where you are kind and help others hoping they will then bebetter off than before and then they will be able to help other people. So if i help 5 people and thenthose 5 peopel help 5 people and those 25 people each help 5 people, the world will be a better place for many people because one person helped 5 people.

"pay back" and "pass it on" = to give back something to someone so that he/she is better off than he/she was before.