What is a flutter apk?

They are the same. Old version of flutter use outputs/apk/release directory but they changed it to outputs/flutter-apk recently . The old directory is kept for compatibility with other building tools.

By default, Flutter builds a debug version of your app.

This tutorial shows you how to build APK files for Android for applications developed using Flutter. APK (Application Package File) is a format used by Android operating systems for distribution and installation . After you build an application, it’s quite common to build APK files to be tested across different devices.