Subject complement:
The word or clause that follows a linking verb and complements, or completes, the subject of the sentence by either (1) renaming it or (2) describing it.

Example:
"Col. Grangerford was a gentleman"
-Adventures of Huckleberry Finn

Explanation:
Gentleman is the subject complement in this example. Huck did this to describe the Colonel in one sentence, so the readers know who Grangerford was. There is no major purpose to this term, other than to describe something, in order for the reader to get an image of something.