CPM is the cost per thousand impressions, and stands for cost per mille. It is used in advertising campaigns that are buying impressions. There are other type of performance-based campaigns that charge per clicks or conversions.
In audio advertising, the CPM is more common though, as listeners and impressions are the direct measure of performance, since the audience is usually not interacting with the device at the same time as the listening happens, so naturally less clicks are recorded. However listeners engage much more with the ads than in display advertising and are generally converting at a later time.
AudioGO currently employs a fixed $18 CPM. This means that we're charging USD18 for every 1000 ads played. This is 1.8c per every impression, which translates into a very effective means of advertising.