As Coleridge explains in Chapter XIV of Biographia Literaria, a good poem is a piece of writing the ultimate object of which is pleasure from individual parts as they are commensurate with the whole, the whole therefore giving equal pleasure as the parts. In other words, a poem is distinguished from prose, such as science and novels, by having both truth and pleasure as its object. And this pleasure must equally be given by the whole as by the constituent parts of the whole.

Therefore a good poet is one who may have an object of truth but must have an object of pleasure and who uses separate parts united together in judicious combination so as to fulfill both the object of truth and the ultimate, requisite object of pleasure derived from the parts as well as from the whole.

To put it a little differently, since a poem tells information as does any piece of prose, the difference between them lies in how the separate parts are combined to convey meaning. For prose, the combination has the sole objective of conveying fact and truth. For poetry the combination has the ultimate objective of conveying pleasure.

In poetry meaning is likewise fact and truth but it is communicated by a different combination of parts--a different arrangement of the same parts used in prose--accommodating also meter and sound and rhyme superadded as desirable. The object that for poetry is superadded above fact and truth is pleasure, pleasure from the different combination of parts and from the whole.

In summary, fact and truth are served but over and above this the object of pleasure is served by each part (each verse, each simile, each image, each combination of words, etc) and by the whole poem. The parts give pleasure equal to the pleasure given by the poem, with the converse being equally true: The whole must give equal pleasure as the parts. A good poet can do these things: say fact and truth; combine parts for pleasure; superadd meter, sound, rhyme; satisfy the objective of pleasure derived from parts and whole.

What is Coleridge's idea of a good poet?

This is an often misunderstood notion of Coleridge and Wordsworth.  Their oft-quoted description -- “the spontaneous overflow of powerful emotion, recollected in tranquility” – from the “Preface to Lyrical Ballads” was an attempt to explain the Romantic poetry impetus for their kind of new poetry, as opposed, for example, to Pope’s rational impetus (and he does criticize Pope).  Coleridge is not criticizing all poetry before them (he had great respect for Shakespeare and the Greek poets, for example), calling one kind of poetry “good” and another “bad.”  Because this collection was large and influential, he wanted to introduce Romantic poetry as a new kind, and therefore tried to isolate the differences in the preface to the collection.  He goes on to describe the connection of the poets to the natural world, and to the connection between humanity and natural energies and “seasons.”  He is criticizing, perhaps, those notions that the poetry comes directly from the “adrenaline” (not his word) of the moment, but from the “recollection” back in the studio” of the moment.  As one scholar put it:  “You don’t stop looking at the daffodils to write a poem about seeing daffodils.”