Which method is called when a Flutter widget is inserted into the widget tree?

The 'initState()' method in Flutter is called when a stateful widget is inserted into the widget tree. It is a lifecycle method that is invoked once when the widget is created. Developers often use 'initState()' to perform one-time initialization tasks, such as setting up controllers or fetching initial data. Understanding the widget lifecycle is crucial for managing state and resources efficiently in Flutter applications.