This little piece of information confuses people too often, dare I say needlessly?
An experienced user will ignore it. So will a beginner, so will most anyone. The difference is an experienced user will be able to tell you what it means, maybe. I say maybe because sometimes explaining it just leaves people more confused.
In the simplest terms, if you see it...that is good, not seeing it...is not. (I know that doesn't help understand why or what it means)
In an attempt to explain it I'll fall back on my library metaphor. We can borrow books from a library, not the shelves. We could try but I bet the librarian would object. We could borrow every book from a shelf, just not take the shelf too. In Revit User Created Worksets are bookshelves (and Revit doesn't care if you take a shelf) and everything in the model is a book. With this in mind these two terms take on the following meaning.
Owner = you take possession of a bookshelf
Borrower = you take possession of a book(s)
(I've got a quick reference too if you're interested.)
In the workset dialog we want to avoid being listed under the Owner column because that means we've taken an entire shelf and all of its books from the library. Imagine the librarian running down the street after you...okay maybe not.
After figuring out what worksets you should create, the most important thing to care about is which workset is the Active Workset. I'd argue that nobody would get confused if (Not Editable) wasn't there at all. The concept of Active Workset is easier to grasp because it parallels the notion of Current Layer in AutoCAD. When you add elements to the model they are assigned to the Active Workset. Simple enough. We can add new elements to a workset even if we are not the owner of the workset. So the notion of editable or not really is a bit too subtle for the typical workflow of Revit users. It's technically accurate but a bit irrelevant.
While I've got the Workset dialog open, I highlighted the Open/Close buttons (item #1). Those define whether Revit loads the elements assigned to the corresponding workset or not. Open means load them up and closed means don't show them anywhere, a global result. Item #2 is for taking ownership of a bookshelf (workset), which isn't necessary or appropriate most of the time. The Editable column next to Owner changes to Yes if you take ownership of the User Created Workset and your username will appear in the Owner column. Don't do that, at least not routinely.
For now, remember that seeing (Not Editable) means you have NOT taken ownership of any User Created Worksets...and that's GOOD. If you don't see (Not Editable) that means you have...and that's NOT good, usually.
Clear as mud?