The Magem David, meaning the "Shield of David", is more commonly known as the Star of David or the hexagram. Though this symbol is internationally recognized as the symbol of Judaism today, it is actually a comparatively recent addition to the religion. In fact, if this symbol appears on any early Jewish art or writing it is cause for immediate suspicion for most anthropologists or archaeologists.
Said to represent an emblem on the Shield of David, or perhaps the shield itself, the Magem David does not seem to be mentioned in early Jewish literature, and does not appear at all until about the medieval ages. It gained its first strong associations with Judaism in about the 17th century when it began to appear on Judaic temples. This symbols ties to Judaism were solidified in 1896, when the Zionist movement adopted it. However, because of its lack of historical usage or biblical support, it remained controversial for quite awhile afterwards.
That being said, it was bestowed with religious significance, both in Judaism and by medieval magicians (though admittedly many Jewish practices were absorbed into European mystical practice). The star is constructed from two intersecting triangles, this is thought to represent the union of god and man in Judaism, and the union of opposites in Ceremonial practice. In addition, this symbol has a history of being thought of as a protective sign. Some Kabbalistic theories hold that the Shield of David was supposed to protect from spiritual advisories, rather than literal flesh and blood ones.
It has also been suggested that this was the symbol King Solomon used to vanquish or control demons, thus it has also been called the Seal of Solomon. However, this appears to be a belief of early Christian mystics, adopted during the medieval ages. Possibly related to the Islamic belief that Suliman (King Solomon) bound the Djinn and Ifrit.