CPI stands for Cost Per Install. This refers to installing an app. An advertiser can use affiliate marketing to promote its app. For this, it can compensate in different ways. The compensation can be based on Cost Per Download. An advertiser then pays when the visitor downloads the app. CPI goes one step further. Only when the visitor downloads and installs the app does the publisher receive compensation.
An alternative to CPI is CPS (Cost Per Sale), where the advertiser only pays when the visitor actually orders something in the app. It often happens that advertisers combine CPI and CPS. This is called a hybrid fee. The advertiser pays a small fee when the app is installed and a percentage fee when the visitor orders something through the app.
Read more about App-install campaigns at Daisycon here