Two of Keats' most famous points of theory on poetry relate to Romanticism and negative capability. Keats did not write a formal theory of poetry; his poetics (theory of poetry) were expressed in letters family, friends and Fanny Brawne. He took exception to Wordsworth's leading Romantic view that poetry is an "egotistical sublime" expression of the poet's self (perhaps this represented a part of Wordsworth's theory that Wordworth was himself unfaithful to without realizing it, e.g., "A Ruined Cottage").
Keats thought this glorification of the individual self much too narrow a scope for the poetic mind to explore and express. Keats countered this theory of Romanticism with the theory of "empathy" whereby the poet submerges her/is senses and perceptions in the experience of another entity or even object (e.g., "Ode to a Grecian Urn") and empathetically conveys its experience and feelings. This corresponds to Keats' theory of negative capability. Keats was an intuitive poet who could exist in the disharmony between perceiving and knowing. This opposes the poetic stance of both Wordsworth and Coleridge who sought rational answers based on knowing (this is true even though Coleridge specialized in combining knowable experience with spiritual meaning (e.g., The Rime of the Ancient Mariner)).
Keats thought of this ability to embrace the ambiguity and mystery between perceiving and knowing as negative capability. Negative capability allows doubt to reign with fact and reason suspended. In addition to this Keats theorized in his letters about imagination, an important concept to poets in the Romantic era. Keats' theory was reminiscent of the classic idea of mimetic poetry in that Keats held that the ideal was revealed through the poet's inspiration.
For Keats, truth was revealed through imagination, and the focus of truth was beauty, and the ideal was born of the synthesis of truth and beauty. This may be why Keats used what is now called synaesthetic imagery, such as in "Ode on a Grecian Urn." Such imagery synthesizes dissimilar sensory perceptions and produces imagery in which plots of green are melodious (color plus sound in a metonymy for grass); song has "sunburnt mirth" (sound is burnt by the suns rays, which requires touch); and light blows on the breeze (sight [i.e., light] has mass and motion).
This theme of synthesis running through Keats' poetic theory is also reflected in his conviction that suffering produces soul; it is soul-making. Keats holds that soul produces imagination, and it is imagination that apprehends beauty, and it is beauty that represents truth, which combine to present the ideal; and it is the ideal that the poet's inspiration reveals. Thus it is that Keats counters and parts from the egotistical self-expression of the Romantic era and reflects classical poetic theory, shone in a new light, in his theory of poetry.
[This answer draws upon the Time Magazine Book Review: The Chameleon Poet (Staff writer); What is nagative capability?, Maureen Roberts; John Keats: An Overview, Lilia Melani, Department of English, Brooklyn College, CUNY; John Keats: Introduction, eNotes.com.]