Basically, auxiliary verbs are function words, a type of closed class which is constituted of words that have a grammatical function as opposed to content words, which are an open class of lexical words. An auxiliary verb is used to add functional or grammatical content to the information expressed by another verb, considered to be the main verb. Auxiliary verbs are also called helping verbs

Examples: The 23 auxiliary verbs

am is are
was were
has have had
do does did
will would
shall should
can could
may might must
Be Been Being