Depends on the school.
At some, like Michigan, everyone gets one.
At schools like UCSF, the secondary really means nothing, just means that you met the basic basic requirements.
At others, like UW, you've been screened and are considered a competitive applicant if you've recieved a secondary, though this doesn't mean that you'll get an interview.
Again at others, like Maryland, it means that an interview invite is on its way.
So it really depends on the school.