Wordsworth's purpose is to explain the aesthetic concepts behind his poetry (and, to a lesser extent, that of Coleridge). In so doing, the Preface articulates how the poetry of the Romantics broke with the classical tradition of seventeenth- and eighteenth- century poetry.

The crucial differences have to do with language and subject. Wordsworth sees his poetry as the "metrical arrangement" of the "real language of men," and he believes that its subject matter should be drawn from "common life." The function of poetry is to reveal the "essential passions" underlying experience and to guide the reader into those habits of mind that enable him to perceive the inherent beauty in nature and everyday life.

Another purpose of the preface is to define the qualities that make a poet, and defend the new poetic forms in Lyrical Ballads from criticism. A poet, Wordsworth says, is a man with heightened sensitivity to the passions of life and, in particular, who is able at times to imagine the feelings of common people as his own. This empathetic ability is combined in the poet with the ability to translate these feelings into language that is both evocative and pleasurable. In other words, the poet, through his sensitivity, is able to capture strong emotion in his poetry and through it make those emotions available to the reader. In this regard, Wordsworth says, the primary business of poetry is the articulation of truth.

The "Preface to Lyrical Ballads" represents nothing short of a revolution in the theory of poetry. In the "Preface," Wordsworth sets out how he thinks poetry should be written as well as providing the reader with a justification of the poetry contained in Lyrical Ballads.

The main idea in the Preface is the notion—radical for its time—that poetry should be written in the language of ordinary men. That being the case, a poet is just “a man speaking to men.” At the same time, Wordsworth seeks to employ his imagination in presenting to the mind ordinary things “in an unusual way.”

In other words, the poems in the collection will be ballads in that they will be like popular songs rooted in the lives of the common people but also lyrical in that they will express personal feelings in a highly imaginative way. As Wordsworth explains, he has chosen what he calls “low and rustic life” as a subject matter for his poems because in that condition, the “essential passions of the heart” can “speak a plainer and more emphatic language.”

On the face of it, there doesn't seem anything particularly groundbreaking about Wordsworth's theory. Yet at the time when he wrote the Preface, the dominant strain of English poetry was based on the notion that poems should be written in an elevated style of language and that they should deal with the great and the good rather than with the lives of ordinary folk.

In the Preface to Lyrical Ballads, Wordsworth wanted to express his theory of poetry. The Preface is therefore a justification of that theory and of the themes and styles of the poems in Lyrical Ballads. 

One aspect of this theory was to use themes about common life (usually in rural environments and situations involving a connection to nature). Thus, Wordsworth wanted to explore how one could attain profound truths and sublime emotional experiences via the imagination. In other words, this process is about understanding the extraordinary while experiencing the ordinary. 

Poetry is to be created out of these extraordinary/ordinary experiences. Poetry will be the spontaneous overflow of emotion reflected in tranquility. The poet has an experience and, reflecting on it later, can arrive at a deeper understanding about that experience and about the act of reflection. The process of experience/feeling and reflection is not just a method for poetic creation; it is also Wordsworth's recommended method for experience in general. 

Wordsworth wanted the style of Lyrical Ballads to stick with the common life theme. He proposed to avoid personification and traditional poetic diction, favoring instead more common (natural) language of people. In a sense, focusing on feeling (lyrical) more than poetic form (i.e., a ballad), Wordsworth shifts the focus from form to content. Although he was attempting a less formalistic poetry in favor of a more natural (even more prose-like) poetry, he did note that verse was the best form for conveying strong emotional content. 

I might perhaps include all which it is necessaryto say upon this subject by affirming, what few persons will deny, that, of two descriptions, either of passions, manners, or characters, each of them equally well executed, the one in prose and the other in verse, the verse will be read a hundred times where the prose is read once. 

One could easily argue that a poem (or a song) has a longer life in the memory than a passage from a work of prose (i.e. a novel). This isn't just because a poem tends to be a shorter work. It's also because of the cadence and rhythm, natural mnemonic devices. In depicting poems about realistic, common people in rustic environments, Wordsworth was rejecting the poetry of the past which tended to treat kings, queens, and heroes in an overly regimented style. For Wordsworth, real people were more relevant. More to the point, Wordsworth believed that sublime emotions can be discovered in the experience and reflection of common experiences.

In other words, it can be inspiring to identify extraordinary virtue in a poem about an extraordinary hero whose exploits are unbelievable to the point of being legendary. Wordsworth supposed that (his main concept) it would also be inspiring, more relevant, and more rewarding to identify extraordinary virtue in a poem about ordinary life.