The Torah tells us:
וְנָתַתָּ אֶל-חֹשֶׁן הַמִּשְׁפָּט, אֶת-הָאוּרִים וְאֶת-הַתֻּמִּים וְהָיוּ עַל-לֵב אַהֲרֹן, בְּבֹאוֹ לִפְנֵי ה
"And you shall place into the breastplate of judgment the Urim and Tumim, and they shall be on the heart of Aharon when he comes before God." (28:30)
Rashi explains that the Urim V'tumim is actually a parchment with the full written name of God that they place in the folds of the Choshen behind the stones. With this parchment, the letters of the tribes engraved in the stones of the Choshen would transmit the word of God to the Jewish people. The Kohen would ask God a question and receive an answer through the letters of the tribes engraved on the breastplate. Without the Urim V'tumim, there was no communication from God. In a sense, the Urim V'Tumim serves as the spiritual batteries of the Choshen.
When the Kohen Gadol asked God a question, how did He send a reply? The Gemara in Yevamos (73a) presents a dispute about this issue. According to Rabbi Yochanan, the letters of the Choshen would protrude in a specific order indicating a message. According to Reish Lakish, the letters would actually move around the breastplate to form a specific text.
Still, either way the Choshen communicated, the rabbis in the gemara noticed a perplexing problem: When you look at all the names of the tribes, the Choshen still lacks two letters from the Hebrew alphabet. Can you figure out what those two letters are?