synchronized(this)里的this什么意思？

看到了一段代码：
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class Zhangsan {
    public void say() {
        System.out.println("张三对李四说：你给我画，我就把书给你");
    }

    public void get() {
        System.out.println("张三得到了画");
    }
}

class Lisi {
    public void say() {
        System.out.println("李四对张三说：你给我书，我就把画给你");
    }

    public void get() {
        System.out.println("李四得到了书");
    }
}

public class Test2 implements Runnable {
    private static Zhangsan zs = new Zhangsan();
    private static Lisi ls = new Lisi();
    private boolean flag = false;

    public void run() {
        if (flag) {
            synchronized (zs) {                                    //此处的zs或者说synchronized () 括号里的东西表示什么意思？
                zs.say();

                synchronized (ls) {
                    zs.get();
                }
            }
        } else {
            synchronized (ls) {
                ls.say();
               
                synchronized (zs) {

                    ls.get();
                }
            }
        }
    }

    public static void main(String args[]) {
        Test2 t1 = new Test2();
        Test2 t2 = new Test2();
        t1.flag = true;
        t2.flag = false;
        Thread thA = new Thread(t1);
        Thread thB = new Thread(t2);
        thA.start();
        thB.start();
    }
}