A verb is a word that tells what the subject of the sentence is being or doing.
Jarrod is upside down.
(The verb is shows what Jarrod is being—is upside down.)
He stands on his head.
(The verb stands tells what he is doing—stands on his head.)
A linking verb tells what the subject is being by linking it to other words. Linking verbs include is, am, are, was, and were.