The Volatile Keyword in Multithreading

Consider the following Java program. What is the primary function of the volatile keyword as applied to the running instance variable?

public class VolatileFlag {
    private volatile boolean running = true;
    
    public void stop() { 
        running = false;
    }
    
    public void runTask() {
        while(running) {
            // perform some task
        }
        System.out.println("Stopped");
    }
    
    public static void main(String[] args) throws InterruptedException {
        VolatileFlag vf = new VolatileFlag();
        Thread t = new Thread(() -> vf.runTask());
        t.start();
        Thread.sleep(100);
        vf.stop();
    }
}