Congratulations, you got asked to go further in the application process. Don't get discouraged if they send all applicants to complete the secondary application. This means you get more essays to set yourself apart from others ( yeah it is more money) but hopefully the end results is that you are closer to an interview.
The BEST advice we got on secondary applications is RETURN them QUICKLY! In fact, many schools have about a two week time frame that they expect them back.
This is all about selling yourself again and showing that you have invested time in looking at the school individually.
Now for 3,000 applications received only about 300 will get interview for 100 students spots. This is discouraging odds. But a great motivator to make sure you stand out and research well.