A complement is a word or group of words that completes the action or state of being expressed by the verb. A subject complement typically follows the verb to be or a linking verb (seem, appear, many others).
Thus, in the sentence you're now reading, the subject complement is the italicized words. Notice that the subject complement never dictates the number of the verb. If the grammatical subject is singular (subject complement), the verb must be singular (is), even if the subject complement is plural (italicized words).
Another kind of complement is the object complement. In the following sentence, the object complement is underlined:
We elected her mayor.