English has three primary auxiliary verbs: do, be, and have. All three take part in the formation of various grammatical constructions, but carry very little meaning themselves. For example, the primary auxiliary be is used to form the progressive, as in: Bill is dancing.
However, it makes very little sense to ask what is means in this sentence. Instead, what is of interest is what is does, i.e. that it helps form a verb phrase which, as a whole, indicates that Bill's dancing is going on at this moment. The same reasoning applies to all the primary auxiliaries. They are auxiliaries in the true sense of being 'helpers' in conveying verbal meaning.