What is API level Android?

What is API Level? API Level is an integer value that uniquely identifies the framework API revision offered by a version of the Android platform. The Android platform provides a framework API that applications can use to interact with the underlying Android system.

What is API level R in Android Studio?

An integer designating the maximum API Level on which the application is designed to run. In Android 1.5, 1.6, 2.0, and 2.0. 1, the system checks the value of this attribute when installing an application and when re-validating the application after a system update.