Cost-per-action (CPA) is a pricing model used in online marketing.
Using CPA the advertiser is only paying the publisher, if the user took a predefined action. These actions can vary in many ways. They can either be a purchase of a good or service, the subscription to a newsletter or in case of a gaming app the completion of a tutorial or certain level within the game.
As long as the advertiser can find an agreement with the publisher, he can define a desired action as complex as he likes. Nevertheless the most common action agreed upon for this pricing model is the purchase of a product or talking about mobile games the completion a tutorial. CPA is especially advantageous for advertisers considering that it reduces the risk of paying for poor traffic quality. Instead it encourages publishers to try to provide traffic that will most likely convert for a minimized ad space.
You can find a detailed overview of all pricing models available in this article.