What is the keyword "this" referring to if it is given as an argument
<p>I know when to use the keyword "this" for fields and constructors but I'm not sure when it is passed as an argument</p>

<pre><code>    import javax.swing.*;
    import java.awt.event.*;

    public class SimpleGui implements ActionListener {
        JButton button;

    public static void main (String[] args) {
        SimpleGui gui = new SimpleGui();
        gui.go();
    }

    public void go() {
        JFrame frame = new JFrame();
        button = new JButton("click me");

        button.addActionListener(this);

        frame.getContentPane().add(button);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 300);
        frame.setVisible(true);
    }

    public void actionPerformed(ActionEvent event) {
        button.setText("I've been clicked!");
    }
}
</code></pre>