Whenever a person reads a poem, they begin to interpret, or make meaning, out of what they read. Some poems are more difficult to interpret than others—perhaps their language is so heightened or abstract that it no longer resembles common speech, or the metaphors, context, or author's point of view are too unfamiliar to the reader for them to gain entry into the poem. Literary criticism can be helpful at these moments to bridge the gap between reader and poet. Interviews, also, can be useful for understanding a poet's intentions.
Poets writing about their own genre can often be illuminating, such as Wordsworth's preface to Lyrical Ballads, T.S. Eliot's "Tradition and the Individual Talent," or Langston Hughes's essay "The Negro Artist and the Racial Mountain." Most of the major writers of the twentieth century also wrote prose about poetry. Or you can start at the present and work backwards: literary criticism includes everything from our journal American Poet to Aristotle. Look up an author you love and see if you agree with the commentary presented on their work. You can do this same thing with an author you dislike or don't understand to see if the insight of others is helpful. Sites like Literaryhistory.com or the Modern American Poetry site are good starting points for background information and critical essays.
Reading reviews can also be a helpful exercise and lend direction to your future reading. Journals like American Poetry Review, Rain Taxi, and The Constant Critic all review contemporary poetry. If you’re interested in formal poetry (sonnets, villanelles, sestinas, etc.), a book of forms can be a helpful tool, providing a description of the form and examples of it for the reader to study and compare. You can also consult a dictionary of literary terms, such as the Princeton Encyclopedia of Poetry and Poetics, to help you define terms like simile, metaphor, and synecdoche.