What is RPM?
RPM or Revenue Per Mille or Revenue Per 1000 Impressions refers to the total revenue generated per 1000 impressions. In this instance, impressions refers to both served impressions and requested impressions.
Why should I care about RPM?
RPM is most important for Publishers. If your RPM is low, you are not getting as much revenue as you could be from the ad places you have available.