The android-apt plugin assists in working with annotation processors in combination with Android Studio. It has two purposes:
Allow to configure a compile time only annotation processor as a dependency, not including the artifact in the final APK or library
Set up the source paths so that code that is generated from the annotation processor is correctly picked up by Android Studio.
This plugin requires the android or android-library plugin (version 0.9.x or up) to be configured on your project.