RPM = rate per thousand uniques

In other words how much you are getting paid per thousand unique visitors.

Example

You send 100 unique visiters to a pay per click advertising site and 10 visitors click thru an ad.
The PPC company pays you $.10 for each of the 10 clicks equalling $1.00 for your 100 visitors.
Based on a thousand unique visitors you should get 100 clicks. 100 clicks at $.10 per click would be $10.00.
So your estimated RPM (Rate per thousand unique visitors) would be $10.00.