Guided Imagery is the process of harnessing your thoughts and using them for your benefit. We all have those moments when we daydream about things. Sometimes they're things we're worried about, like a test we're having soon. Other times they're things we're happy about, like that person we have a crush on. Whether these thoughts are positive or negative, they come into and out of our minds all the time.
Guided Imagery helps you to choose which thoughts you're going to have and when, and to direct them to go the way you want them to, so that your unconscious mind will know what your life will be like when you achieve your goals. If you're trying to quit smoking, you can guide yourself through an image of your typical day without having to have a cigarette. If you're concerned about an upcoming job interview, you can imagine yourself acing it and impressing your soon-to-be boss.
Your unconscious mind is far more willing to cooperate with what your conscious mind wants when it has a chance to feel what this new experience will be like. Guided Imagery gives you a chance to provide that. Since you're always thinking about something anyway, why not grab hold of those thoughts and use them to your best advantage?