The “atonement cover” (NIV) formed the gold top of the ark of the covenant.
Exodus 25:17-22 (TNIV)
Make an atonement cover of pure gold—two and a half cubits long and a cubit and a half wide. And make two cherubim out of hammered gold at the ends of the cover. Make one cherub on one end and the second cherub on the other; make the cherubim of one piece with the cover, at the two ends. The cherubim are to have their wings spread upward, overshadowing the cover with them. The cherubim are to face each other, looking toward the cover. Place the cover on top of the ark and put in the ark the tablets of the covenant law that I will give you. There, above the cover between the two cherubim that are over the ark of the covenant law, I will meet with you and give you all my commands for the Israelites.
The translation notes for the NET give the following information:
The noun is כַּפֹּרֶת (kapporet), translated “atonement lid” or “atonement plate.” The traditional translation “mercy-seat” (so KJV, ASV, NASB, NRSV) came from Tyndale in 1530 and was also used by Luther in 1523. The noun is formed from the word “to make atonement.” The item that the Israelites should make would be more than just a lid for the ark. It would be the place where atonement was signified. The translation of “covering” is probably incorrect, for it derives from a rare use of the verb, if the same verb at all (the evidence shows “cover” is from another root with the same letters as this). The value of this place was that Yahweh sat enthroned above it, and so the ark essentially was the “footstool.” Blood was applied to the lid of the box, for that was the place of atonement (see S. R. Driver, Exodus, 269-270).
The atonement cover was where the blood was sprinkled on the Day of Atonement (Leviticus 16:14-15). In the New Testament, Jesus Christ is described as our “atonement cover” using the same word as was used in the Greek translation of Exodus 25:
God publicly displayed him at his death as the mercy seat accessible through faith. This was to demonstrate his righteousness, because God in his forbearance had passed over the sins previously committed. (Romans 3:25 NET)
(Strangely, the NET uses the traditional “mercy seat” here and in Hebrews 9:5 but not in any Old Testament passage.)
Translators struggle with this passage because it presents Jesus as both the source of the blood and the place where the blood was sprinkled. The ISV makes the mixed metaphor obvious:
Romans 3:24-25 ISV:
By his grace they are justified freely through the redemption that is in the Messiah Jesus, whom God offered as a place where atonement by the Messiah’s blood would occur through faith.