Coleridge is remembered for his theory of imagination as set forth in his book Biographia Literaria, a kind of literary memoir. Coleridge divided the imagination into two parts: primary and secondary.
Primary imagination has to do with interpreting the real world; it is the "the living power and prime agent of all human perception." Secondary imagination, on the other hand, is creativity or the ability to see how ideas connect and form new constructs; it:
dissolves, diffuses, dissipates, in order to recreate: or where this process is rendered impossible, yet still at all events it struggles to idealize and to unify.
In this sense, the poetic imagination is a kind of reflector or interpreter of Nature or of the divine.
Coleridge's other famous concept has to do with his famous description of reading as the "willing suspension of disbelief." That is, in encountering a poetic work, the reader must suspend their tendency to question the logic of the piece and enter into a kind of "poetic faith" in the imaginative power of the writer. This idea—that the reader enters into a kind of shared imaginative space with the writer, and that literature is not meant to be read like an instruction manual—has been extremely influential in criticism to this day.