I think what is meant is a principal verb: A verb of more than one word is called a verb phrase. A verb phrase contains a principal verb and one or more auxiliary verbs. So in the sentence, I have been eating sandwiches for a week , eat is the principal verb.
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A principal verb is the mainspring of a sentence. They can stand on their own in a sentence. They don't need the help of another verb to make a sentence meaningful. A principal verb can be in present or past tense forms.
For example; Be (am, is, are), Have (has, have), Do (do, does) verbs are called principal verb.