ANSWER: The object you are asking about is referred to in several ways in the KJV Bible. It is called the Ark of the Covenant (Numbers 10:33, Deuteronomy 31:26, etc.), of the Testimony (Exodus 30:6), of the Lord (Joshua 4:11,1Samuel 4:6), of God (1Samuel 3:3, 17, etc.) and of God's strength (2Chronicles 6:41).
The Ark itself was a very special container God had Moses build. It contained the stone tablets (given to Moses) which delineated the Ten Commandments (on which the covenant was built), the rod of Aaron the High Priest that budded, and a pot containing manna. Its first mention in the Bible is in Exodus 30.
The Ark was the holiest physical object located inside the holiest place in God's temple. Placed in an area commonly referred to as the 'Holy of Holies' or the 'Most Holy Place,' it represented the very throne of God.
The room where this object resided was so special that only the temple's High Priest, once a year (on the Day of Atonement or Yom Kippur), could enter into it. If anyone else tried to go into the room at any time, or even if the High Priest entered it at any time other than to perform a special service on Atonement, they would be killed by God (Leviticus 16:1 - 2).
Revelation 11 states that this object will be seen when the third heaven, the place where God's throne resides, is opened (Revelation 11:19).
The object you reference was very likely destroyed when Nebuchadnezzar leveled Jerusalem, including the temple of Jehovah, in 586 B.C. We find an interesting statement in Jeremiah which declares that the ark and all that is with it will NOT be important during the millennium (Jeremiah 3:16).
It may well have been, shortly before or after the words of Jeremiah 3:16 were written, that the ark was destroyed by the Babylonian army, along with the temple. Revelation's reference to it seen in heaven is not the physical one God gave Moses to build. The book of Hebrews, chapter 9, explains that the spiritual or holy relics (if that is the right word to use) found in Jerusalem's Temple were patterns or copies of spiritual realities in heaven (Hebrews 9:1 - 5, 23 - 24).
The Ark of the Covenant, as revealed in Revelation 11:19, is not the physical object once found in the temple and transported by God to heaven. This verse, as a side note, offers indirect evidence that the Fourth Commandment is still in force today. It is implausible to believe an annulled weekly rest day would still have a spiritual reality representing it in heaven long after Jesus' death and resurrection!