What does the term 'imagery' mean in poetic language?

Imagery is actually a very simple concept.

Poetic imagery occurs any time the poet uses the five senses: sight, hearing, smell, taste or touch.  

If you can see in your mind what the poet has written, that is a visual image. For example, if the poet describes yellow leaves dangling on a tree branch, you can imagine what that looks like, so that is a visual image. If the poet describes a sound in a person's head as a shrill, screaming bird, you can hear that, so it's auditory imagery. If the poet describes the scent of incense in the air, you can smell that. Often imagery is so commonplace that your eyes might brush over it, but something as simple as "her cheeks were red" is an image, because you can see those red cheeks in your mind's eye. So if an instructor would like you to find imagery in a poem, simply find everything the poet describes that you can see, hear, touch, taste or smell.