In "A Defence of Poetry" Percy Bysshe Shelley is (1) defining poetry and (2) creating a defending apologetics of sorts for the poets and poetics of the Romantic Age. The poets of the Romantic Age and their poetry broke with poetic tradition on two important points. Firstly, since at least Aristotle's definition of poetics through to Sir Philip Sidney's re-articulation of Aristotelian poetics right up to the Romantics, poetry was believed to be a mimetic inspiration from God and meant to instruct on how to live by divine precepts, instruction that the human heart craved for. As such, poets believed their role was to show how to live, love and be as they were inspired to realizations of mimetic truths by God. Romantics believed that poetry was inspired by the natural order and processed through the poet's imagination, which analyzed the relationships of things, and was a self-expression instead of an expression of a mimesis of a heavenly principle. As a result, they focused on reflecting how people did live instead of instructing in how to live.
Secondly, again since at least Aristotle's time poetry required high subject matter of great importance with highly placed protagonists of great influence and high poetic diction. By contrast, the Romantics embraced subject matter that was commonplace, e.g., Wordsworth's The Ruined Cottage, having common characters and protagonists, with low language of common people (Coleridge rejected the notion that it was even possible to write poetry in common language since all poetic utterances were filtered through the poet's imagination, Biographia Literaria). As a result of these departures from ancient poetic tradition, Romantic poet's had critics who disliked and disapproved of the new turns poetry had taken. It was against these detractors that Shelley was defending the poetry of the Romantics.
Shelley defines poetry as the mind at work through the power of analytical imagination upon thoughts produced by the faculty of synthesizing reason. Reason "enumerates" the "qualities" of the objects of thought while imagination perceives the relationships and value of those objects of thought. Shelly begins with and sums up his essay by concluding that poets are the "prophets" and "legislators" of society by virtue of their role as "authors of language ... institutors of laws ... founders of civil society ... inventors ... [and] teachers."