non-manifold topology polygons have a configuration that cannot be unfolded into a continuous flat piece. Some tools and actions in Maya cannot work properly with non-manifold geometry.
Two-manifold topology polygons have a mesh that can be split along its various edges and unfolded so that the mesh lays flat without overlapping pieces.
Research
https://blender.stackexchange.com/questions/7910/what-is-non-manifold-geometry
Ciaran :3 