An auxiliary verb is a helping verb that comes before the main verb in a sentence. Some examples of auxiliary verbs are: do, does, did, have, has, had, be, am, is, are, was, were, can, will, shall, could, would, should, may, might, and must. The document provides examples of how these auxiliary verbs are used in sentences, such as "I have eaten my food" or "I am going to the park." It explains that auxiliary verbs are followed by different forms of the main verb, such as the base form, past participle, or present/past participle.