Sentence cores must have subjects and verbs. Some sentence cores also have completers. Completers “complete” the idea expressed in the core. Sentences with active and linking verbs usually have completers, though not always. Sentences with passive verbs seldom have completers.
In this sentence, “Jake ordered shrimp,” Jake is the subject, ordered is the verb, and shrimp is the completer. Shrimp tells what Jake ordered. It completes the idea expressed by the subject and the verb.