Exact meaning unsure; probably part of the fortifications of the City of David, often identified with the so-called "stepped stone structure" on the east.
About Millo
A fortress or defensive wall that was part of the city of David in Jerusalem (2 Samuel 5:9; 1 Chronicles 11:8). King Solomon later rebuilt or made this fortification bigger (1 Kings 9:15; 11:27).
Two kings of Judah had important connections to the Millo:
Some of Joash's servants killed him in “the house of Millo” (2 Kings 12:20).
Key References
So David took up residence in the fortress and called it the City of David. He built it up all the way around, from the supporting terraces inward.
All Scripture References (7)
2 Samuel (1)
So David took up residence in the fortress and called it the City of David. He built it up all the way around, from the supporting terraces inward.
1 Kings (3)
This is the account of the forced labor that King Solomon imposed to build the house of the LORD, his own palace, the supporting terraces, and the wall of Jerusalem, as well as Hazor, Megiddo, and Gezer.
As soon as Pharaoh’s daughter had come up from the City of David to the palace that Solomon had built for her, he built the supporting terraces.
and this is the account of his rebellion against the king. Solomon had built the supporting terraces and repaired the gap in the wall of the city of his father David.
1 Chronicles (1)
He built up the city around it, from the supporting terraces to the surrounding wall, while Joab restored the rest of the city.
2 Chronicles (1)
Then Hezekiah worked resolutely to rebuild all the broken sections of the wall and to raise up towers on it. He also built an outer wall and reinforced the supporting terraces of the City of David, and he produced an abundance of weapons and shields.
Isaiah (1)
You saw that there were many breaches in the walls of the City of David. You collected water from the lower pool.