Mujarrad means that the lexical form contains no additional morphological letters beyond its radicals. Mazīd fīh includes one or more additions.
FeatureUnaugmentedAugmented
Root lettersPresentPresent
Extra lettersNoneOne or more
Example from the bookجَعْفَرٌجَوَاسُوسٌ
جَعْفَرٌ — جَوَاسُوسٌ
Translation: Jaʿfar, a personal name; spies.
Visible length alone is not proof of augmentation; first establish the underlying radicals.