In order to write any Dart program, be it a script or a Flutter app, you must define a function called main.

void main() {
  print('Hello, Dart');
}

This function tells Dart where the program starts, and it must be in the file that is considered the "entry point" for you program. By convention, this will be in a file called main.dart.

The main function can execute any code within it's code block that you'd like it to.

You must have exactly one main function in your program.

In general, your main function will likely initiate any pre-processing needed for your program, and end with a function call that runs the rest of your app. Some examples:

Main function in simple printing script

void main() {
  // calls to separate function
  // which handles main functionality
  printGreeting();
}

Main in a Flutter app

void main() {
  // any preprocessing can be done here, such as determining a device location
  //
  // runApp is a Flutter function that runs your Flutter app
  runApp(MyApp());  
}

Main function in an AngularDart app

void main() {
  // runApp is also used in AngularDart
  runApp(ng.AppComponentNgFactory);
}