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I have a general idea but can someone explain exactly what it is.



Here's the simple explanation...

It's where "Joe Jerkoff" signs up to get paid to send traffic to a website, where they signup (or convert). Joe sends the traffic, they signup but the website operators shave the number of signups to screw Joe out of the compensation the website is obligated to pay under the terms of the agreement. Joe has no real way to tell if he's being screwed though, because it is the website that is keeping the tabs.