The concept of "pay it forward" has been around for many years, at least since the times of ancient Greece. However, the principles contained therein remain true today. In short, it involves passing the payment for a debt "forward" to another person, rather than to the original lender. The next person is expected to do the same, onward and onward, theoretically spreading the goodness forward to many people.
My personal encounters with "pay it forward" have come through the tea and fountain pen communities. In the case of the tea community, these are more often trades or giveaways, rather than true "pay it forwards," but there are certainly instances where quantities of tea, such as large numbers of samples, were sent to those, who were new to the tea community, or were sent from one person to the next, with each person taking a little and adding a little to each shipment.
However, it is in the fountain pen community that I have seen the best examples of "paying it forward." Never before have I encountered a group of people, who are so willing to share and support those with fewer resources or those, who are merely new to the community. Oftentimes, people will post "pay it forward" offers on sites such as The Fountain Pen Network, volunteering to send pens, ink, and other fountain pen paraphernalia to others, the only stipulation being that, someday in the future, when the person is able, the recipient "pay it forward" by offering some sort of related good to someone else, regardless of the method or medium.
"Pay it forward" is a huge leap of faith in the good that is humanity. Yet, the chance to start a chain of good that helps a number of people is completely worth the time and effort that it takes to go out of one's way to help another person.