In Parhsat Tetzaveh (Shmot 28:29-30) we read:

Aharon will carry the names of B’nai Yisrael on the Choshen HaMishpat (breastplate of judgment), over his heart, when he comes into the Holy sanctuary, as a constant remembrance before God. Place in the Choshen the Urim v’Tumim, and they will be over Aharon’s heart when he comes before God. Aharon will carry the judgment of B’nai Yisrael over his heart, before God always.

The Torah does not explain exactly what the Urim v’Tumim are.

According to Rashi, the Urim v’Tumim is the inscription of the Explicit Name (written on parchment) which was placed into the folds of the choshen.

Ramban (Shmot 28:30) agrees with Rashi’s statement and adds:

The proof for this is that in the work of the craftsmen, the Urim v’Tumim is not mentioned at all. It says in Shmot 39:2, 8: “And he made the ephod…and he made the choshen” but it does not say “and he made the Urim v’Tumim.”

It only says “Place in the choshen the Urim v’Tumim” since they were not the work of craftsmen. Neither craftsmen nor B’nai Yisrael had any part whatsoever in the making of this donation, for they were a secret transmitted by God to Moshe and he wrote them in holiness.

The Urim v’Tumim were the holy Names of God, and it was by virtue of the power residing in these Names that the letters inscribed upon the stones of the Choshen would light up  before the eyes of the of the kohen who inquired of their judgment.

We see in Bamidbar 27:21 that on account of this inscription it is called judgment: “Before Elazar, the Kohen shall he (Yehoshua) stand, who shall inquire for him the judgment of the Urim before God…”

When they inquired in Shoftim 1;1: “Who shall go up (ya’aleh) first to fight against the C’naanim?” the kohen fixed his thoughts on those Divine Names which were on the Urim (lights) and the letters forming the name of the tribe of Yehuda lit up before his eyes. For the word ya’aleh (go up), the letter “yud” lit up from the word Levi, the “ayin” from Shimon, the “lamed” from Levi etc. The Tumim (perfection) helped the Kohen figure out the sequence of the letters.

The Mishna in Yoma 71b gives us some background on what the Kohen Gadol (High Priest) needs to wear and how the Urim v’Tumim are used:

The Kohen Gadol serves in eight vestments and the common Kohen in four. A common Kohen serves in a tunic (kutonet), breeches (michnasaim), a turban (mitznefet) and a belt (avnet). The Kohen Gadol, in addition to these, wore another four vestments: The choshen, the ephod, the robe (meil) and the tzitz. Dressed in these eight vestments the Kohen Gadol is consulted for the discussion of the Urim v’Tumim. And they would not consult them on behalf of just anyone, but only for a king, or on behalf of someone that the public needs.

How do the Urim v’Tumim work?

The answer is explained in Yoma 73a:

The one consulting (the king or the head of the Sanhedrin) faces the one being consulted (the Kohen Gadol), and the one being consulted faces the Shechina (Divine Presence). The one consulting says: “Shall I pursue this troop?” and the one being consulted responds: “Thus said God: Go up and triumph!”

When the wars of Israel were fought in the Land of Israel, religion and state were united. This concept is totally different from how the rest of the world works where there is a separation of religion and state

The Jerusalem Talmud, Yoma 7:3 asks:

Why are they called Urim?

They are called Urim (light) because they were enlightening Israel.

Why are they called Tumim?

They were called Tumim (complete) because they were straightening out the way before them. As long as B’nai Yisrael were following the straight path, they would be shown the correct way.

Imagine if we had the Urim v’Tumim today where we could be shown by God how to fight this war!