Your teacher probably wants you to give specific examples of the imagery used in the play and then tell how the use of this imagery adds to the play. Why does the playwright use the imagery? Imagery appeals to our five senses: taste, touch, smell, sight, hearing. What does the playwright use in the play to evoke an image in our head that lets us know what something in the play looks like, tastes like, smells like, feels like, or sounds like? That's what is meant by imagery.
For example, there are many references to decay in the lives of the Hunsdorfers that represent the theme of death in the play. The setting is the front room of the Hunsdorfer home, a rundown, wooden building that used to be a vegetable shop. The clutter of the house represents the broken bits and pieces of Beatrice's dreams. Just as she's trapped in this room during the whole play, she's trapped in these circumstances of her life. Think about the marigolds--what do they represent? Only through her marigolds does Tillie experience any growth and have the courage to pursue her dreams.
Once you get your examples of imagery that are used in the play, you can then tell how these examples make the play better or how they add to the themes of the play. I hope this helps. For more information, go to the links below. Good luck!