Flutter is a Dart Framework for building mobile 2D Apps for iOS and Android.
The Flutter SDK provides utilities, functions, and widgets to create apps with one codebase.
In short words, Flutter is a framework with a set of features that use Dart as a programming language to interact with the Flutter SDK.
The Flutter mindset is like build apps as widgets tree and everything should be a widget.
We will use our Flutter API with Dart for creating Apps and the Fluter SDK take our Dart code to transform into native code for IOS and Android.
Flutter looks nice, I expect to play more in the future, If you want to learn more about flutter, you can take the following online courses, I recommend first read a little bit about dart language, take online the first app and then continue with the Udacity Flutter course.