While Wordsworth is not setting out a complete poetic defense wherein he defines his aesthetic ("I have therefore altogether declined to enter regularly upon this defence"), it is true that in the Preface he does discuss his ideas of what the poet is, what poetry is and, most importantly to Lyrical Ballads, what the language of poetry is.
Wordsworth first implies that a Poet is one who arranges language expressing ideas in metrical form. This language he arranges is "in a state of vivid sensation." In other words, it is emotional reaction to ideas or emotional expressions of feeling. The Poet is one who "rationally" (i.e., reasonably) imparts the "vivid sensation," or emotionalism, in metrical form to readers. In other words, in Wordsworth's view, the Poet discerns vivid emotional states in people around him and captures those emotional states in poetic meter and rhyme ("metrical arrangements") to "impart" this vision to the reader.
Wordsworth also has something to say about what a Poet is not. This false Poet substitutes "feeling ..., philosophical language" with "arbitrary and capricious habits of expression" that falsely accrues honor to their poetic skill. In other words, these anti-Poets turn their backs on the real language of everyday expressions of emotion and invent "their own creations" of poetic language that are artificial, the product of a whim, lacking philosophical importance, lacking clarity. They think they become honored poets this way but really only "furnish food" for poor taste that has no solid bearings and is "fickle."
Regarding Poetry, Wordsworth implies that Poetry is in part a matter of what is customary:
[The reader will] struggle with feelings of strangeness and aukwardness: they will look round for poetry, and will be induced to inquire by what species of courtesy these [Lyrical Ballads] can be permitted to assume that title.
More importantly though, Poetry is the outflowing of emotion--of sentiment--that has been tempered by serious, "long," deep thought and that therefore describes "objects" and "sentiments" that relate to "important subjects" of discussion.
Poetic language, according to Wordsworth--and this is one of the paramount ideas in the Preface--is the language of common people speaking everyday expressions and expressing everyday sensations of rural (i.e., pastoral) people in an [idealized] rural life. Wordsworth says this language is "more emphatic" having "greater simplicity" being "more accurately contemplated" and "incorporated with the beautiful and permanent forms of nature." In other words, commonplace, "low," "rustic" language is more intuitive, insightful, genuine, sincere than is the contrived elegant language of Poets (or anit-Poets).
Importantly, Wordsworth concedes that this language must be (1) filtered through the Poet's mind and (2) cleaned up, "purified," of what is vulgar, crass, incorrect and offensive before this shinning quality of greatness can show through. Thus many critics, including Coleridge, have found great contradiction and fallacy in Wordsworth's position on poetic common, low, language.
The language, ... [must be] (purified indeed from what appear to be its real defects, from all lasting and rational causes of dislike or disgust)