Before we begin, it's important to note that there is a distinction between descriptive detail and imagery.
Imagery is rich detail that is designed to appeal to and awaken the senses. We generally refer to imagery by its specific type, such as visual imagery.
To explain further, let's consider this sentence.
The girl sat in front of the fire.
There's nothing really wrong with this sentence except that it provides only the most basic descriptive detail we need to understand what's happening: there's a girl, she sat down, and there's a fire. However, this sentence is not at all rich or immersive. If I asked you to visualize this sentence, what would you imagine? Is she sitting on a floor? On a chair? For that matter, is she indoors or outside?
Visual imagery gives us the specific detail we need to create a vivid picture in our mind. It tells us that the teenager threw herself down so hard on the leather couch in front of the fireplace, or that the five-year-old crossed her legs on the worn carpet at her grandmother's feet, ready for story time. Imagery ensures that the reader gets involved in the story.