As above, so below is called an occult maxim. The word occult means "hidden," and the word maxim means "truth," or "underlying principle." When Witches seal a circle casting, sacred space or a spell, they often say the words "As above, so below. This circle (or spell) is sealed. So mote it be." That's great but what does this mean? The words As above, so below stem from the mystery traditions of Alchemy, and specifically from an amazing piece of work called the Emerald Tablet.
Wicca emphasizes the immanence of divinity within Nature, seeing the natural world as comprised both of spiritual substance as well as matter and physical energy. Many Wiccans also embrace the idea of the spiritual transcendence of divinity in planes of spiritual existence outside the physical world. This combination of transcendence and immanence allows for the intermingling and the interaction of the unmanifest spiritual nature of the universe with the manifest physical universe; the physical reflects the spiritual, and vice-versa.