Setting is the key to many stories. I Capture the Castle by Dodi Smith would not be nearly so powerful if it was set elsewhere. The Great Gatsby would be a completely different story if it took place in the Midwest. So many stories rely on their location to tell the story.
This exercise is one of creating a solid setting. Think of a place you would like to write about, then find a photo of it. Maybe it will be a photo you took, or one you found on the internet.
Focus on the photo but don’t describe what is going on in it, instead add details to it, expand from it, try and flesh out the world. Use it as a jumping off point for your imagination.
Don’t try to add characters or a plot, just imagine the details of the land itself. Describe the weather there, the temperature, the types of trees, and ferns. Or perhaps it is a desert and you have to describe it in terms of dust, sky, and sand.
This exercise is all about imagining the environment. Try and spend at least ten minutes describing it on paper. After the first ten minutes you can do anything, you can save the location for later, you can add people, you could even throw the exercise away. No matter what you do from there, you will have improved your ability to create a detailed setting.