What does external mean in Dart?

9.4 External Functions
An external function is a function whose body is provided separately from its
declaration. An external function may be a top-level function (17), a method

The body of the function is defined somewhere else.
As far as I know this is used to fix different implementations for Dart VM in the browser and Dart VM on the Server.

When we make an external function inside a class like toString()

external String toString();

means this method is abstract and the child of the parent class will add the function body, that's because in Dart we only can make an abstract class.

Summary:

external function = abstract function in not abstract classes