What is the purpose of declaring the ‘stop’ variable as volatile in the above program?

public class Flag {
    public volatile boolean stop = false;
}

public class Worker extends Thread {
    private Flag flag;
    public Worker(Flag flag) { this.flag = flag; }
    public void run() {
         while (!flag.stop) { }
         System.out.println("Stopped");
    }
}

public class Main {
    public static void main(String[] args) {
        Flag flag = new Flag();
        Worker worker = new Worker(flag);
        worker.start();
        try { Thread.sleep(100); } catch(Exception e) {}
        flag.stop = true;
    }
}