Simple Definition:

A verb, in English grammar, is a word that describes an action, occurrence, or state of being. It expresses what the subject of a sentence is doing or what is happening to the subject. Verbs can also show tense (past, present, or future) and can be changed to match the subject of the sentence. For example, in the sentence "She runs every day," the verb "runs" describes the action of the subject "she." In the sentence "He is happy," the verb "is" shows his condition.

Very Simple Definition:

A verb is a word that tells what someone or something is doing. For example, in the sentence "He runs," the word "runs" tells us what he is doing. In the sentence "He is happy," "is" shows how he feels.