in wow what does unique equipped mean?

Answer 1

That you can only wear one exemplar of that item at a time. For example, you can wear two identical white vendor trash daggers, but most purple are unique, and you therefore need two different daggers instead.

Answer 2

Rings and trinkets come with the "Unique-Equipped" restriction to prevent a player from wearing two of the same item.  However, if it was simply "Unique" you couldn't keep two of the same items for different specs (gems/enchants/reforge) so it being Unique-Equipped allows you to do that.

Its just their fix for solving the problem that they had in the ToGC instance where people would wear one normal and one heroic version of the various rings and trinkets.