A subject complement is the adjective, noun or pronoun that follows a linking verb.
The complement of an intransitive verb always says something about the subject and is therefore called subject complement
In simple words the adjective, noun or pronoun which complements the subject is the subject complement
Mrs. Wallace is my
Mrs. Wallace is the subject
(is) is the linking verb
teacher (the predicate noun) complements the subject that is Mrs. Wallace thus teacher is the subject complement.
Subject complement cannot be a verb
When I woke up, I felt
I is the subject
felt is the linking verb
Sick complements the subject thus is the subject complement