Inside the breastpiece of decision you shall place the Urim and Thummim, so that they are over Aaron’s heart when he comes before the LORD. Thus Aaron shall carry the instrument of decision for the Israelites over his heart before the LORD at all times.
Moshe inserted a parchment with the Ineffable Name of God into the pouch that was created, by folding the Choshen in half. This was called the Urim, from the word ohr/light.
The Urim would illuminate individual letters on the stones of the Choshen in order to convey Hashem’s messages. It was also called the Tumim, from the word Tomim/complete, because if the message was read in a correct and complete order, it would convey Hashem’s answers to the Cohen.
Towards the end of the First Temple era, King Yoshia removed the Urim veTumim from the Choshen and hid it along with the Aron Kodesh and the oil of anointment.
The Urim Vetumim had another name: “Kreti uPleti”. Kreti from the word “Koret/to cut - to be decisive” and Pleti, from the word “Pele/wonderous”, because the Urim Vetumim were“wondrous” in their abilities.