Non Manifold stuff
 

When you do a cleanup and you tick non manifold edges so you can convert to sub D. What are non-manifold edges and how do you fixt them?

non-manifold geometry, going off the maya documentation definition, is "Non-manifold [geometry] polygons have a configuration that cannot be unfolded into a continuous flat piece [that is, when it is unwrapped for texturing]".
things like three faces sharing an edge, and suchlike.
In the case of edges, its usually where a single edge is just standing out in space, without an associated face (like the result from an 'Extrude Vertex' or similar).

Whats the best way to fix it. Do you have to make it into a four sided poly?

er... the best way to fix it is to do clean up and select the non manifold option. ;)