Predicate adjective:
One type of subject complement-an adjective, group of adjectives, or adjective clause that follows a linking verb. It is the predicate of the sentence and modifies, or describes, the subject.

Example:
"Col. Grangerford was very tall and very slim, and had a darkish-paly complexion... he had the thinnest kind of lips, and the thinnest kind of nostrils, and a high nose, and heavy eyebrows, and the blackest kind of eyes."
-Adventures of Huckleberry Finn

Explanation:
Huck describes Col. Grangerford using the predicate adjective. He does this to easily describe the Colonel, quickly and swiftly. The reader gets a good mental image of what Col. Grangerford looks like, which is Hucks main purpose.