Singleton Design Pattern Double Checked Locking

if (searchBox == null) { //1
synchronized (SearchBox.class) {
    if (searchBox == null) {  //2
        searchBox = new SearchBox();
        }
    }
}

}

here is my custom class for singleton pattern. in this code, I use double-checked locking as above. As I read many posts on some source, they say that double-check is useful because it prevents two concurrent threads run at same times make two different objects. As per threading concept, at a time an only single thread gets executed by thread scheduler. Then how 2 threads will try to execute the above code.

Please explain to me. What have I understood wrong?

Thanks :)