Hello world in Flutter

main.dart

// Importing the material library with common UI components
import 'package:flutter/material.dart';

// Entry point for our application
// It tells Flutter to run the app and use TODOApp widget as the entry point
void main() => runApp(TODOApp());

// The root widget for our app
// It is stateless, that is why it expends StatelessWidget
class TODOApp extends StatelessWidget {
  
  // We are overriding the build method
  // It has to be done for every custom widget you define
  @override
  Widget build(BuildContext context) {
    // Material app is a built in widget to help you with the app layout
    return MaterialApp(
      title: 'TODO app',
      // Scaffold is a built in widget to help you with the specific screen layout
      home: Scaffold(
        // AppBar is the header in the top of the screen
        appBar: AppBar(
          title: Text('TODO app'),
        ),
        // This is the actual content of the screen
        // We output the text 'Hello word', also centering it in the middle of the screen
        body: Center(
          child: Text('Hello world')
        )
      )
    );
  }
}