In JavaFX, the ProgressBar class represents a control that displays a progress bar. You can use it to display the progress of a task, such as a file download, by updating its progress property.

Here's an example of how to create and use a ProgressBar:

import javafx.application.Application;
import javafx.concurrent.Task;
import javafx.scene.Scene;
import javafx.scene.control.ProgressBar;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class MyJavaFXApp extends Application {

    @Override
    public void start(Stage primaryStage) {
        // Create a ProgressBar object
        ProgressBar progressBar = new ProgressBar();
        
        // Create a VBox and add the ProgressBar to it
        VBox root = new VBox(progressBar);
        
        // Create a scene and set it on the stage
        Scene scene = new Scene(root, 300, 250);
        primaryStage.setScene(scene);

        // Start a task that updates the progress of the ProgressBar
        Task<Void> task = new Task<Void>() {
            @Override
            protected Void call() throws Exception {
                for (int i = 1; i <= 100; i++) {
                    updateProgress(i, 100);
                    Thread.sleep(50);
                }
                return null;
            }
        };
        
        // Bind the ProgressBar's progress property to the task's progress property
        progressBar.progressProperty().bind(task.progressProperty());
        
        // Start the task
        new Thread(task).start();
        
        // Show the stage
        primaryStage.show();
    }

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

In this example, we create a ProgressBar object, and add it to a VBox object. We then create a Task object that updates the progress of the ProgressBar, and bind the ProgressBar's progress property to the Task's progress property using the bind() method. Finally, we start the Task and show the Stage.

As the Task updates the progress of the ProgressBar, the ProgressBar will automatically update its appearance to reflect the current progress. In this example, the Task updates the progress every 50 milliseconds, until it reaches 100%.