How is MockResponse used by MockServer while Unit Testing?

I am using MockServer and MockResponse as follows -

@Before
public void startup() throws Exception {
    server = new MockWebServer();
    server.start();
}

@After
public void tearDown() throws Exception {
    server.shutdown();
}

@Test
public void http_Response() throws Exception{
    server.enqueue(new MockResponse().setResponseCode(500));
    ...
    ...
    assert(getErrorCode(), 500);
}

I do not understand how mockResponse object is used internally by MockWebServer and why it is used in that fashion? (At a higher level, I am trying to understand why my asserts are failing, but I will get to that later) Tried searching documentation and tutorial, but there does not seems to be lucid explanations out there.