These words circulate throughout occult and magical circles, and they come from Hermetic texts. The concept was first laid out in The Emerald Tablet of Hermes Trismegistus, in the words “That which is Below corresponds to that which is Above, and that which is Above, corresponds to that which is Below, to accomplish the miracles of the One Thing.”
In accordance with the various levels of reality: physical, emotional, and mental, this relates that what happens on any level happens on every other. This is however more often used in the sense of the microcosm and the macrocosm. The microcosm is oneself, and the macrocosm is the universe. The macrocosm is as the microcosm, and vice versa; within each lies the other, and through understanding one (usually the microcosm) you can understand the other.