CPA stands for Cost Per Action.
These are offers where you want your visitor to leave their personal information, you want them to take action. This could be name and an email address or it could be more than that, say income + phone number for example.
Companies then use the data to contact the individuals for certain offers. You get paid for collecting the data for them.
These offers are used by universities for example, collecting personal information so that they can contact potential students.
Some CPA offers can be boarder line scammy, make sure you look into the offer and know what it`s all about.