CPM - Cost Per Thousand Impressions
When an advertiser buys ads on a CPM (Cost Per Thousand Impressions) basis, they pay for every 1,000 impressions. This means an advertiser will pay even if there is no action taken on the part of the user (action being a like or a view, for instance).
An impression is the total number of times content is displayed in a person's timeline feed - whether it is engaged with or not.
For example: User A is served an ad one (1) time, and User B is served an ad three (3) times. That would equal four impressions.