Question

Subject complements are words that

Answer 1

follows a linking verb and describes or changes the subject of the sentence.

Answer 2

As the name itself says, subject complements are words which complete a subject. They are always preceded by a linking verb in a sentence (a linking verb connects two parts of a sentence). Here is an example of a subject complement:
He is a good boy.
The subject is HE; the linking verb is IS; and the subject complement is BOY.