In MED, I create a model, simple cylinders and blocks. I move them close together to make a table with legs. When I use the tools option to test the mesh, I see I have 9 objects, 6 closed, 3 open. Could someone explain to me:

1. what does this mean
2. is it a problem
3. how do I correct or avoid it?

Occasionally I will end up with a model that has all objects open. Using Lithunwrap to weld the model seems to resolve most of those objects, but not all of them. Since I am not sure of what I am doing exactly, it makes me a little uncomfortable. I really am ignorant of the issue here, I know. But I would like to come to an understanding of this. ;-)

Thanks!