Among the garments of the High Priest, the breastplate and the Urim and Tumim were perhaps the most enigmatic. The breastplate was a patterned brocade made from gold thread, blue, red and crimson wool and twined linen. It was set with four rows of precious stones mounted in gold settings - twelve in all. The names of the twelve tribes were engraved on the stones - one on each stone - as well as the names of the Patriarchs, Abraham, Isaac and Jacob. According to the Zohar, the Urim and Tumim were the Forty-two and Seventy-two Letter Names of G‑d placed in the folds of the breastplate that caused the letters engraved in the stones to light up in sequence in order to spell out an answer to a question asked by the High Priest.

Place the Urim and the Tumim into the breastplate of judgment and they shall be over Aaron's heart when he comes before G‑d. (Ex. 28:30)

Rabbi Yehuda said: The meaning of the word "urim" has been explained as deriving from "meirim" [meaning "illuminating" - since it illuminated the letters engraved on the stones of the breastplate]. This is the mystical secret of "the lens that shines".

Malchut is called 'the lens that does not shine', since it does not have its own illumination….

The Aramaic term for "the lens that shines" is "aspaklarya meira". The term "aspaklarya" has been translated as glass, mirror, looking-glass, speculum, telescope etc. The basic meaning is that it is the medium through which Divine Inspiration is focused and manifested. (This may be the origin of the "crystal ball" in non-Jewish fables.) The commentary Mikdash Melech relates this to the cluster of sefirot called Zeir Anpin, formed by the six sefirot that are illuminated by the Forty-two Letter Name.

This "lens that shines" is formed by the letters of the holy Forty-two Letter Name lodged there in the folds of the breastplate through which the worlds were created.

The Tumim is the mystical secret of the letters included within "the lens that does not shine" ["aspaklarya sh'eina meira", i.e. the sefira of Malchut1] that is illuminated by the Seventy-two Letter Name engraved on it.

Their power illuminated the other letters that were engraved on the stones of the breastplate….

Malchut is called "the lens that does not shine", since it does not have its own illumination. Instead it receives all its illumination from Zeir Anpin. Thus the two are compared to the sun, the source of light, and the moon, which has no light of its own. Nevertheless, malchut is illuminated by the loftier Seventy-two Letter Name, which has its source in chochma, according to the principle of "the father [chochma] establishes the daughter [malchut]". (Ziv HaZohar)

They are the mystical secret of the Holy Name. Together they are called "Urim and Tumim".

Come and see: when those letters of these two names were lodged there [in the fold of the breastplate] their power illuminated the other letters that were engraved [on the stones of the breastplate] - i.e. the letters of the names of the twelve tribes (see Yoma 73b) - lighting up some and leaving others dark.

And thus the High Priest was able to find the answer to his question and act accordingly.

[Zohar II, p. 234b; translation and commentary by Moshe Miller]