Your professor is referring to the main image which is to be found in most poems. You can find discussions of many poems with central images in eNotes. For example, in Robert Frost's heavily discussed "The Road Not Taken," the central image is a place where two roads diverge in the woods. In Shakespeare's heavily discussed Sonnet #55, the central image is a cemetery full of tombstones and monuments. In Edgar Allan Poe's heavily eNoted poem "The Raven" the central image is that of a raven sitting on top of a bust of the goddess Pallas Athene. In John Keats's heavily eNoted "Ode to a Skylark" the central image is a skylark singing in some bushes.
It is pretty easy to find the central image and then to see why and how the poet is using it. In Wordsworth's famous poem "The Daffodills" the central image is a field full of golden daffodills. In Keats's "On First Looking into Chapman's Homer" the central image is a book containing a translation of Homer. In Robert Frost's "Stopping by Woods on a Snowy Evening" the central image is a stand of trees being covered with snow. In Robert Browning's famous "My Last Duchess," also heavily covered in eNotes, the central image is a portrait of a beautiful young woman. Hart Crane wrote a famous poem about the Brooklyn Bridge. Guess what the central image was.
I suggest that you read about some of these poems in eNotes and familiarize yourself with the concept of central images. From there, it should be easy to discuss why and how poets use them. The central image often serves as the inspiration for the poem itself, as with Wordsworth's "Daffodills." In other cases the central image serves as a metaphor for something the poet is experiencing or re-experiencing, as with Poe's raven.