People become mortal again, but they age at an accelerated pace that is possibly based on how long they have lived as vampires.
At least, this is what I get from what happened to Katherine in tonight's episode.
My theory is that it's because the Cure was only made so that immortals can be killed and pass on, so if nobody kills them, they eventually die out naturally not long after having taken the Cure.
If I'm right, then this will become more of a race against the clock for Silas, Qetsiyah, Amara and Katerina.
In all cases, I think we should get an explanation from Qetsiyah, if she has one. After all, she was the one who made the Cure. How she made it and what exactly is the nature of the Cure are mysteries that still need to be solved.
What do you guys think?