Imagery is just like having a picture in your head - but you don't need a camera or a screen to do it. It's what you see, feel, taste, smell or hear in your mind when you think about something.
For example, when you think about going to the beach, you might imagine the bright blue ocean, the squeaky sand under your feet, the salty smell of the sea air, the warm sun on your skin, and the sound of seagulls chirping.
It's like a movie in your head, but you get to control everything. Imagery helps you remember things better and understand what something would be like without actually being there. It's also used a lot in books and stories to help you picture what's happening.