In the beginning of chapter 14 of the Biographia Literaria, Coleridge discusses how he and Wordsworth came up with their groundbreaking book of poetry Lyrical Ballads. Coleridge writes:
During the first year that Mr. Wordsworth and I were neighbours, our conversations turned frequently on the two cardinal points of poetry, the power of exciting the sympathy of the reader by a faithful adherence to the truth of nature, and the power of giving the interest of novelty by the modifying colours of imagination.
As Coleridge explains, Wordsworth's poems in that volume focused on the first "cardinal" or main point of poetry: being faithful to nature or what we would call today realistic. Coleridge's fewer poems in the volume were dedicated to expressing the second cardinal point of poetry: modifying the imagination by focusing on the times when the unexpected or supernatural break into the life of a person. Coleridge mentions his poems The Rime of the Ancient Mariner and Christabel as examples of this second cardinal point.
As Coleridge points out, he and Wordsworth were interested in conveying through verse "the dramatic truth of such emotions" that the supernatural or seemingly supernatural experiences have on people. He notes that most people at least once in a while have an experience that seems uncanny or eerie or outside of the ordinary, so this seemed a fit subject for poetry.
A key point for the two men that united the seemingly opposite types of poems--the simple everyday poems Wordsworth wrote about his emotional reaction, say, to seeing a field of daffodils waving in the wind versus Coleridge's poems about fantastic, supernatural events--was the adherence to emotional truth. They wanted to capture how people really respond to such events, putting realistic emotion at the center of their endeavor.
Finally, Coleridge defined the poet as one who brings "novelty and freshness" to his subjects, conveys "a more than usual state of emotion, with more than usual order," and who, while his poetry "blends and harmonizes the natural and the artificial," "subordinates art to nature."
These ideas were written about earlier by Wordsworth in his Preface to the Lyrical Ballads, in which he discusses the great aim of poetry being "emotion recollected in tranquility." Coleridge would end up having sharp differences from Wordsworth, but they both agreed on the central importance of emotional truth in poetry.