In the old ages not all the people knew how to communicate with God, not all the people were chosen to be prophets, not all the people knew what was the intention of the Almighty....
In order to solve the problem there were three kinds of solutions: the first is asking the prophet; the second is to pray for God; the third is using the the Urim and Thumim with the help of the priest.
The Scriptures doesn't help us to understand what are they exactly, but we know that they help to discover the willing of God. The first appearance of them we can find in
Ex 28:30 as written:

" וְנָתַתָּ אֶל-חֹשֶׁן הַמִּשְׁפָּט, אֶת-הָאוּרִים וְאֶת-הַתֻּמִּים, וְהָיוּ עַל-לֵב אַהֲרֹן, בְּבֹאוֹ לִפְנֵי יְהוָה; וְנָשָׂא
אַהֲרֹן אֶת-מִשְׁפַּט בְּנֵי-יִשְׂרָאֵל עַל-לִבּוֹ, לִפְנֵי יְהוָה-תָּמִיד"

"And thou shalt put in the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron's heart, when he goeth in before the LORD; and Aaron shall bear the judgment of the children of Israel upon his heart before the LORD continually."

The high priest took the Urim and the Thummim in his pocket, near to his heart, physically and also metaphorically. The question is what they are. Are they stones, cards, notes, sticks? No one knows but I think that it doesn't matter. There function is important, to reveal the light, the truth, the one who is cursed and who is righteous and innocent.
We can see a usage of them in Joshua 7:14- 18

"וְנִקְרַבְתֶּם בַּבֹּקֶר, לְשִׁבְטֵיכֶם; וְהָיָה הַשֵּׁבֶט אֲשֶׁר-יִלְכְּדֶנּוּ יְהוָה יִקְרַב לַמִּשְׁפָּחוֹת, וְהַמִּשְׁפָּחָה אֲשֶׁר-יִלְכְּדֶנָּה יְהוָה תִּקְרַב לַבָּתִּים, וְהַבַּיִת אֲשֶׁר יִלְכְּדֶנּוּ יְהוָה, יִקְרַב לַגְּבָרִים. וְהָיָה, הַנִּלְכָּד בַּחֵרֶם--יִשָּׂרֵף בָּאֵשׁ, אֹתוֹ וְאֶת-כָּל-אֲשֶׁר-לוֹ: כִּי עָבַר אֶת-בְּרִית יְהוָה, וְכִי-עָשָׂה נְבָלָה בְּיִשְׂרָאֵל. וַיַּשְׁכֵּם יְהוֹשֻׁעַ בַּבֹּקֶר, וַיַּקְרֵב אֶת-יִשְׂרָאֵל לִשְׁבָטָיו; וַיִּלָּכֵד, שֵׁבֶט יְהוּדָה. וַיַּקְרֵב, אֶת-מִשְׁפַּחַת יְהוּדָה, וַיִּלְכֹּד, אֵת מִשְׁפַּחַת הַזַּרְחִי; וַיַּקְרֵב אֶת-מִשְׁפַּחַת הַזַּרְחִי, לַגְּבָרִים, וַיִּלָּכֵד, זַבְדִּי. וַיַּקְרֵב אֶת-בֵּיתוֹ, לַגְּבָרִים; וַיִּלָּכֵד, עָכָן בֶּן-כַּרְמִי בֶן-זַבְדִּי בֶּן-זֶרַח--לְמַטֵּה יְהוּדָה.

" In the morning therefore ye shall draw near by your tribes; and it shall be, that the tribe which the LORD taketh shall come near by families; and the family which the LORD shall take shall come near by households; and the household which the LORD shall take shall come near man by man And it shall be that he that is taken with the devoted thing shall be burnt with fire, he and all that he hath; because he hath transgressed the covenant of the LORD, and because he hath wrought a wanton deed in Israel........ And he brought near his household man by man; and Achan, the son of Carmi, the son of Zabdi, the son of Zerah, of the tribe of Judah, was taken.

The one who took from the booty is Achan. After the usage of the Urim and the Thummim, we all know who it is.....

Main phrases of the post + transcription + translation

Hebrew       Transcription        Translation
אָרוּר            'ārûr                   Cursed
אוּרִים וְתֻמִּים   'ûrîm wetummîm Urim and Thummim
רָצוֹן                rātsôn               Will
שָׁלָל                šālāl                 Booty
חֹשֶׁן               hōšen               breastplate
כִּיס                 kîs                    Pocket

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