While Plato suggests that poets are irrational and ignorant, Shelley considers poets to be speakers of universal truth and conveyors of the motives of human nature. The large topic of how Shelly defends poetry can only be briefly touched upon in this format, but this will get you started.
Shelley defends poetry by asserting that "poetry is a mirror which makes beautiful that which is distorted." Shelley's point here is that poetry is "the very image of life expressed in its eternal truth." Shelley upholds the mimetic principle espoused by Aristotle and contends that poetry produces analogies to things of life through imagery that participates in mimesis, thus also participates in "the life of truth":
words unveil the permanent analogy of things by images which participate in the life of truth;
Shelly further defends poetry by attributing to poets "almost superhuman wisdom" and likening poets to nightingales who sing in darkness and illuminate human, emotional, psychological "darkness" by the sweet truth and sound of poetry. Shelly states that there is divinity in poetry that works at a depth beyond that of consciousness.
[poetry] acts in a divine and unapprehended manner, beyond and above consciousness
The defence of poetry as the sweet song of truth by a nightingale might have a forced fit with today's poetry, however Shelley has anticipated this in further defence of poetry. He stresses that each poet, though a master of the traditionally prescribed form of poetry, must adapt his poetic song and harmony to his particular versification in his particular era.