Cost-per-tap is an Apple Search Ads advertising model where you pay for every tap on your banner. The trick about CPT is that you don’t earn from the tap itself. You earn from what happens after.
After users tap on your ad, they find themselves on an app store product page, then (if things shape up well) they download, install and open your app. Only upon these actions users may make an in-app purchase, buy a subscription or view native ads.