In JavaFX, a Timeline is a class that represents a series of key frames that define a sequence of changes over time. It is used for creating animations and other time-based effects in JavaFX.

To use a Timeline in JavaFX, you first create a Timeline object and specify one or more KeyFrame objects that define the changes you want to make over time. Each KeyFrame specifies a time duration and a EventHandler that is called when that duration has elapsed. You can also specify an interpolation method to use between key frames, which determines how the values are calculated between each key frame.

Here's an example of how to use a Timeline to create a simple animation that changes the color of a rectangle over time:

import javafx.animation.Animation;
import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;
import javafx.util.Duration;

public class TimelineExample extends Application {

    @Override
    public void start(Stage primaryStage) {
        // Create a rectangle
        Rectangle rectangle = new Rectangle(100, 100, Color.RED);

        // Create a Timeline with a KeyFrame that changes the color of the rectangle after 2 seconds
        Timeline timeline = new Timeline(new KeyFrame(Duration.seconds(2), event -> {
            rectangle.setFill(Color.BLUE);
        }));

        timeline.setCycleCount(Animation.INDEFINITE);

        // Create a StackPane layout and add the rectangle to it
        StackPane layout = new StackPane(rectangle);

        // Create a Scene with the layout and set it on the stage
        Scene scene = new Scene(layout, 300, 200);
        primaryStage.setScene(scene);

        // Start the timeline
        timeline.play();

        // Show the stage
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

In this example, we create a red rectangle and a Timeline that changes the color of the rectangle to blue after 2 seconds. We set the cycleCount of the Timeline to Animation.INDEFINITE so that it will continue running indefinitely. We create a StackPane layout and add the rectangle to it. We then create a Scene with the layout and set it on the stage. Finally, we start the Timeline by calling the play() method on it.

You can customize the Timeline by adding more KeyFrame objects, adjusting the duration and interpolation method, and specifying what happens when the Timeline finishes by using the various methods of the Timeline class.