what is poetic imagery?

It means “figures of speech”. The most common ones are metaphor (an implied comparison, such as “she is a rose”) and simile (= a direct comparison, such as “she is like a rose”).

Metaphor is said to be an implied comparison because the poet does not use the words “as” or “like”. If he says that “she is a rose”, it means that she has the qualities of a rose (delicate, pretty, etc.).

Simile, on the other hand, is a direct comparison because the words “as” or “like” must be used in the comparison between two terms (“she” and “the rose”).

It is imagery that evokes a response from the reader, as opposed to graphic imagery. For example, if you said the temperature was 80 degrees below zero, that doesn’t mean a whole lot. It’s just a number. But if you said your breath froze into crystal droplets that sprinkled on the ground as you exhaled, or the cold bit into your lungs, preventing you from inhaling, it gives the reader a much more vivid picture of what you are talking about.