Highlight on targeted mob.
if the nameplate was already showing before targeting it (getting in range of the mob), it will have a yellow glow to indicate it is your target. if you target a mob in the distance, the nameplates wasn't showing because of range, the glow won't be there when you get near the mob and your still targeting it.
the glow is something remaining from the blizzard nameplates? Can't find anything in the new Nameplates code from Tukui. The way it is now it's somewhat confusing when it will show or not.