Impressions are the first step of your acquisition journey; the number of impressions means how many people saw your ads. At the advent of the internet, ads were sold & bought by the number of impressions mimicking the offline world.
As the internet progressed and tracking capabilities developed, the model switched to a payment per clicks which in the mobile world translates into taps. Hence the concepts of Tap Through Rate (TTR) and Cost per Tap (CPT).
As the name indicates, the TTR is the number of people who “tap” on your ad to the people who see it. And the total money spent on ads, divided by the number of users who tap on your ad gives the CPT. Make sure to check out this article to improve the performance of your ads.
Today, there are a multitude of ad channels that propose different pricing strategies: some are based on CPT, some based on CPI and some are even based on CAC. However, the deeper towards the subscriber you go, the more difficult it is to track the user especially after the introduction of Apple’s ATT. So make sure you compare your various channels with the same metrics and beware of how they self-report their performance: always double check your numbers with Apple’s.
Search results on the AppStore are also a main driver of traffic to your app page and just like Google you can optimize (AppStore Optimization: ASO) or bid to appear in results (Apple Search Ads: ASA).
ASA also has its own rules & tricks and should be done well to optimize your budget. See an example here.