What is an incremental download?
This could refer to one of two things, installing an app in the first place or updating an app that is already installed.

There are also a number of techniques that developers can use to reduce the size of their apps. One that was introduced in iOS 9 is App Thinning. This is where a developer tells the App Store which parts of their app are necessary all the time and which parts are optional. For example with a game, a developer might just choose to provide a small number of levels initially, with subsequent levels to be downloaded as and when the player reaches them. This reduces the initial install size and storage size used on your device. You can read about App Thinning and other techniques that developers use to reduce the size of their apps here: Technical Q&amp;A QA1795 - Reducing the size of my App.

As of iOS 6 when you install an app update on your device it might just  download the files that have actually changed within the app. The idea being that these "update packages" should be a smaller than downloading another complete copy of the app. For example with a game, if the update is just to fix a crashing bug it might just need to update a single executable file rather than downloading all the interface files, images, level information, etc that make up an app. You can read more about this here: Technical Q&amp;A QA1779 - Reducing Download Size for iOS App Updates