Community promotion ads are ads that promote other SE sites, external web sites, or proposed Stack Exchange sites that are currently in Area 51.
As an example, here's the ads either used or proposed for Skeptics SE.
I'd like to promote some proposed Stack Exchange sites, and I assume there are various useful internet resources that could do with some advertising.
The procedure for getting community promotion ads is basically the SE deciding it wants such ads, which is what this question is about.