Topology knowledge needed

[Khangai]

What is meaning of the highlighted edge on that? And why its unable to delete ? Pls share some experience.
On the other side, there is no such edge. http://i.imgur.com/O4hmem9.jpg

[Gen]

It's probably a border edge (look at the object in smooth preview) so you might have some merging/welding to do before you can get rid of it.