Guided imagery is form of focused relaxation or meditation. Some people have difficulty stopping/slowing a noisy mind or spiraling thoughts that lead to difficulty sleeping, anxiety, and stress. Guided imagery focuses on a positive and relaxing image that is intended to calm the listener down. The guide should be pulling all the listener's senses into the image for the best results so that the listener can almost feel they are a part of the image and they are fully focused on the image. The listener becomes so enveloped in the image that the previous disturbing/stressful thoughts are quieted.
Some practiced people can guide themselves to an image, but there are many options where you can buy or listen to guided imagery for free (ie youtube) if you need assistance.
Studies have shown that people who practiced guided imagery have decreased stress, anxiety, and even depression. The focused image can also take the mind off of pain and decrease stress that often triggers more pain (such as headaches). Those who have issues sleeping from a busy mind often become so focused on the peaceful image they find it easier to sleep.