In Flutter, the Key is an identifier for StatefulWidgets and Elements. It allows Flutter to uniquely identify a Widget and, in some cases, associate that Widget with a RenderObject.

Keys are used in several different situations in Flutter:

• When a StatefulWidget is rebuilt, Flutter compares the new widget’s key to the previous widget’s key. If the keys are the same, Flutter assumes that the widget is unchanged and will simply reuse the existing Element to hold the new widget’s state. If the keys are different, Flutter will create a new Element for the new widget.
• When the parent of a StatefulWidget is rebuilt, Flutter will try to find the child widget in the new tree using the child widget’s key. If it finds the child, it will reuse the existing Element to hold the child widget’s state. If it does not find the child, it will create a new Element for the child.
• When building a ListView with ListTiles, it’s important to give each ListTile a unique key so that Flutter can preserve the state of the tiles when the list is scrolled.

Here is an example of how to use a Key in Flutter:

class MyWidget extends StatefulWidget {
  final String value;

  const MyWidget({Key key, this.value}) : super(key: key);

  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  // ...
}

In this example, the MyWidget class has a Key called key and a String called value. The MyWidget class is a StatefulWidget, so it has an associated _MyWidgetState class that holds its mutable state.