This simple statement is at the core of many metaphysical schools of thought, including the popular practice of astrology. Though it is a central principle of Hermeticism, the meaning has a great deal of relevance in many other ideas.
The meaning of the words is quite literal, though the original quote is longer. The energies in the universe around us have a direct impact on our mundane Earthly lives. The movement of planetary bodies can effect larger energy fields, which is why those movements have a "trickle down" effect on our own personal energy fields.
The specific phrase comes from the larger writings that have been attributed to Hermes Trismegistus (Hermes Thrice Great) who is a figure that seems to be a composite of Greek Hermes and Egyptian Thoth. The study and following of these writings is known today as Hermeticism or Western Hermetic Tradition, though I will not dig too deeply into that practice at this time (though I likely will in the future because it really is very interesting).
An emerald tablet containing 14 statements about the nature of the world is one of the main texts in Hermeticism. The rest of Hermes' writings involve many concepts of alchemy, astrology and the occult. The Emerald Tablet, as it is generally called, has been translated many times over the centuries.
One translation of the full "as above, so below" line is this:
"That which is below is like that which is above, and that which is above is like that which is below to do the miracles of one thing"
But there is more relevant material as well:
"It ascends from the earth to the heaven again it descends to the earth and receives the force of things superior and inferior"
This also seems to imply a direct relationship between earthly energy and planetary energy. Anyone with an interest in astrological fields should take some time to further study this material and see how it relates to more modern studies of astrology and cosmic theory.
Though the history of these statements doesn't really mean much in terms of how accurate they are, it is quite a curious set of teachings nonetheless. Studies of astrology is more recent times does seem to prove out the theory of "as above, so below". We have all seen the personality similarities with people born under the same sign, and how events in our lives mysteriously mimics the movements of planets that are so far away. Perhaps these mystical truths were once understood in the past as well.
Diane is an avid student of astrology, astronomy and other similar fields of interest. She enjoys teaching others about these subjects and she is currently exploring the idea of changing your astrological sign by influencing your astro-quantum field. YourAstroChange.com has more information on the concept.