This illustration comes from Eliphas Levi's 19th century publication Transcendental Magic. In it, he describes it as: "The great Symbol of Solomon. The Double Triangle of Solomon, represented by the two Ancients of the Kabalah; the Macroprosopus and the Microprosopus; the God of Light and the God of Reflections; of mercy and vengeance; the white Jehovah and the black Jehovah."
There's a lot of symbolism packed into that explanation. The Macroprosopus and Microprosopus translate to "creator of the greater world" and "creator of the little world." This, in turn, can refer to a number of things as well, such as the spiritual world and the physical world, or the universe and the human being, known as the macrocosm and the microcosm. Levi himself states that the Microprosopus is the magician himself as he shapes his own world.
As Above, So BelowThe symbolism is also frequently equated to the Hermetic maxim "As above, so below." That is to say, things that happen in the spiritual realm, in the microcosm, reflect throughout the physical realm and the microcosm. Here that idea is emphasized by the literal depiction of reflection: the dark Jehovah is a reflection in liquid of the light Jehovah.
Hexagram – Interlocking TrianglesThis can also be compared to Robert Fludd's illustration of the universe as two triangles, with the created universe being a reflection of the spiritual trinity. Fludd uses triangles specifically as a reference to the trinity, but the hexagram – two interlocking triangles, as used here – well predates Christianity.
PolarityLevi's own description emphasizes the 19th century occult view stressing the interaction of opposites in the universe. Besides the duality of the spiritual and physical worlds, there is also the idea of there being two sides to Jehovah himself: the merciful and the vengeful, the light and the dark. This is not the same as good and evil, but the fact is if Jehovah is the creator of the entire world, is omnipresent and omnipotent, then it stands to reason he responsible for both good and bad outcomes. Good harvests and earthquakes were both created by the same god.
OuroborosThe snake eating it's own tail that surrounds this image is an ouroboros. Ouroboros can represent totality, completion, eternity, and regeneration, among other things.
Read more: Gallery of Historical Ouroboros Images