There are a few main reasons why you may want to hide a registration class from public view:
1. Create a special registration price option for a select group of participants (e.g. sponsors)
2. Let certain people sign-up prior to the official launch of registration
3. Set up transfer classes to allow people to transfer their registration
Once you hide a registration class, the only way for potential registrants to "unlock" the class is for them to enter a specific Invitation Code (which you'll need to create).
In the meantime...
HIDE (or unhide) A REGISTRATION CLASS
Pooof! It's hidden. Or unhidden, as the case may be.
Now that you've successfully hidden the Class, it's a great time to create the Invitation Code(s) to unlock it.
Oh, and here's a video of the entire process for all you visual folks: