Why we use Callback response in enqueue method of OKhttp in android

I just made a simple JSON parsing program with Okhttp now what is Callback in Response of OKhttp and Why we use this ?

OkHttpClient okHttpClient=new OkHttpClient();

    Request request=new Request.Builder().url(url).build();

  okHttpClient.newCall(request).enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {

        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {

        }
    });

• Callbacks are used for asynchronous calls, in this case it will either return you the result from your network call to the url(Json/Xml data) in onsuccess or if there is an error onfailure will be called.

• Error could be if connection didn't got through or connection timeout, response timeout, resource/address not valid, etc.