When using Mokito, what is the difference between the actual object and the mocked object?

In the program below, I am trying to use mockito with junit in my test case. But I don't see how Mokito is helping to create objects for my test? I don't see anything special here as it seems as if mokito is instantiating the actually object.

   public class TestCase1{

    @Mock 
    MyClass myClass;

    public void setup(){

       MokitoAnnotations.initMoks(this);

       }

       public void testAddition(){

         when(myClass.add(2,2)).thenReturn(20);
         assertEquals(4,myClass.add(2,2));
      }
   }

Is it mocking an object, the same as injecting(DI) an object? I appreciate your help!