Why need two IF in singleton pattern in the multiple threads scenario

Refer to http://blog.163.com/xhy_cy/blog/static/1049762320112305016599/

    private readonly string connectionString;  // only can be initializable once 
    private static object syncRoot = new Object();//为了解决多线程，而添加的一个附加变量，以避免死锁

    public static SConnection Instance{
        get{
            if (instance == null){//第一个进程和第二个进程同时走到此，判断都为true
                lock (syncRoot){//第一个线程锁定，在释放前不允许其他线程进入，只能等待
                    if (instance == null){//作用是只允许实例化一次，避免再次被实例化
                        instance = new SConnection();
                    }
                }
            }
            return instance;
        }
    }