Hello.
using Maya 8.5 on Precision 690
Windows XP Pro

I am trying to convert a poly I have created in to a subdiv. When I try to do this I am given an error message directing me to the script editor. Below is the explanation and correction given:

"Nonmanifold geometry cannot be converted to a subdivision surface.
// To clean up nonmanifold edges, use Mesh->Cleanup with the nonmanifold option."

When I do this my poly switches to component mode with certain edges selected and the problem is solved.

What is "nonmanifold geometry" and how do I avoid creating it?

Thank you.
T