A complement is a word or phrase that is essential to the meaning of the sentence.
Casey and Rebecca are teammates.
Rebecca gets angry.
Notice that without the complements the meaning of the sentences would be incomplete.
Casey and Rebecca are
Complements can be phrases or clauses:
Casey is in a good mood.
The problem is that Rebecca is too sensitive.
More than one complement joined by and is called a compound complement.
Casey and Rebecca are friends and teammates.
Rebecca gets stressed and angry.