The sigil of Lucifer first appeared in the Grimorium Verum. It symbolizes Lucifer, as well as his energy, and can be used in various esoteric practices.
There is so much to say about Lucifer, the bearer of light, and therefore, metaphorically, of knowledge. Depending on one's beliefs, he can be seen as a God, as an egregore or as a symbol of our own quest for knowledge and spiritual awakening through work. In Luciferian Wicca it is seen as the God, the male energy of the universe.