Explain with reference to William Wordsworth's theory of language: "Poetry sheds no tears such as angels weep, but natural and human tears she can boast of no celestial choir that distinguishes her vital juices from those of prose, the same human blood circulates through the vein of them both."

In answering this question it's important to understand the context in which Wordsworth and Coleridge wrote Lyrical Ballads. At that time, in the late eighteenth century, English poetry was expected to conform to certain standards of taste and decorum. For the sake of convenience, this kind of poetry has been classified by literary critics and historians as neoclassicism, as it sought to maintain and uphold the standards of classical culture as expressed in the verse of Ancient Greece and Rome. Neoclassical poetry tended to be very formal, closely following tried and tested rules and methods of composition. Its style was somewhat elevated and artificial, its language far removed from the kind spoken by ordinary people in their everyday lives.
Wordsworth and Coleridge set out to change all that. Wordsworth in particular believed that the distinction between poetry and prose was artificial. As far as he was concerned, there was no good reason why poetry and prose couldn't be written in the same style and use the same kind of language. If poetry could break free from its neoclassical straightjacket, it would then be in a position to talk about ordinary people and their lives in a language which they could understand.
At the time, this was considered a radical notion. According to the prevailing standards of taste, ordinary people were not fit to form the subject matter of poetry; their lives were considered too unimportant. Instead, it was thought that poetry should be written about great men and women and their deeds, about colorful figures from antiquity such as gods and goddesses, nymphs, dryads, and wood sprites. And as the subject matter of poetry ought to be elevated, so too should be the language in which it was expressed. Neoclassical poetry was no place for ordinary people or the language which they spoke.
That's what is being driven at in the above excerpt. Poetry should concern itself with the emotions of real people, living, breathing human beings, instead of with larger-than-life characters from history and figures from classical mythology. Only in this way can poetry reach out to a wider audience and deal with the experience of common folk, ignored for so long by the literary elite.