"Pay it forward" means instead of paying BACK the person who helped you, help someone else by doing them the same (or similar) kindness that your benefactor did for you, thus "paying your debt forward" to another needy person.

It means that if someone does something nice for you, then you should do something nice for someone else in turn. So that the kindness and service keeps moving forward.

There's a very good film called Pay It Forward, try to find it. Trailer: http://youtu.be/qfW0wCV9iFI

@Gary, yes ! Thanks Gary ! I get it. If I get help from someone and I will help others too who are in needs of help. Thank you !

Susanne and Lee have given good answers. To add to/clarify those both, the suggestion with 'pay it forward' is that the person you 'pay it forward' to also pays it forward to someone else, and then that person does the same, and the same... That way, one 'good turn' becomes a string of 'good turns'.