Pay it forward

Context #1  

John: Have you heard what people have started doing at the Starbucks drive through?  The person in front is paying for whatever the people behind them have ordered.

Steve: Yeah, I've heard about that happening. I guess people are just trying to be kind to each other.

John: That's right. It's a really cool way to pay it forward.  

Context #2

Jenny: Look someone left their wallet in the classroom.

Christine: What should we do?

Jenny: Let's take it to the school's office. I know they have a lost and found. Maybe they can even find the owner.

Christine:That's a good idea. I'm glad you're doing the right thing and paying it forward.

Meaning: "pay it forward" is an idiom that has come into use relatively recently.  It is used when someone does something kind or morally right knowing that if they do good, then in the future someone else will be kind or do good to them. If you are kind, you will receive kindness in the future. If you do bad to others, something bad will happen to you in the future. 

http://languagesystems.edu/