Cleanup?
 

Okay, going from Poly to Sub-divs, what exactly does Cleanup (non-manifold edges) do?

And once it hightlights whatever it is... does that mean there is a mistake in there that needs to be fixed? Or is that just what was "cleaned up"? Thanks all!

when you have have to do a cleanup its means that one of your normals is pointing the wrong way.

nonmanifold means that something in ur model is disrupting the continuity...most likely in the area it highlighted, you have and extra face sticking out somewhere that doesnt need to be...or u have one face intersecting with another, but not connected to it. See if u can clean that area up a bit...if u don't u wont' be able to smooth or convert.

All righty, I think I get it. Thanks very much!