PTP - Paid to Promote - Can Someone Explain How it Works?

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Well, I know I'm not stupid but I can't seem to wrap my brain around how PTPs work. I know I belong to a number of sites which have statements such as, "..get paid to promote our site.." and I know that I click on a bunch of pages which read, "..user blahblahblah just got paid for showing this page.." but once I look into it and see things like 80 CPM, and all those decimal places, and tier countries and such, I'm lost!! I'm wondering whether someone can give a kindergarten level explanation of how it works and whether it's a good way to make additional money. I don't know whether I'm hitting my own personal math-related "black hole," but I'm just not understanding it. Thanks much in advance.