Auto Dagger2 is an annotation processor built on top of the Dagger2 annotation processor.
It basically generates the component for you.
The goal is to reduce the boilerplate code required by Dagger2 when you have “empty” or simple components. It is usually the case in Android development.
You can also mix manually written components with the ones generated by Auto Dagger2. Auto Dagger2 produces the human-readable code you would (hopefully) write yourself.