Stale value of Shared variable

While reading Concurrency in practice I read that:

NoVisibility demonstrated one of the ways that insufficiently synchronized programs can cause surprising results: stale data. When the reader thread examines ready, it may see an out of date value. Unless synchronization is used every time a variable is accessed, it is possible to see a stale value for that variable. Worse, staleness is not all or nothing: a thread can see an up-to-date value of one variable but a stale value of another variable that was written first.

public class NoVisibility {
    private static boolean ready;
    private static int number;

    private static class ReaderThread extends Thread {
        public void run() {
            while (!ready)
                Thread.yield();
            System.out.println(number);
        }
    }

    public static void main(String[] args) {
        new ReaderThread().start();
        number = 42;
        ready = true;
    }
}

I am not getting meaning of stale. Both threads are sharing the same reference, how can a value modified by one be or not be visible to other thread?