The Hebrew word means "the fill." It was a series of terrace walls built on a steep slope, supporting the fill behind it in order to create level areas. Houses were built on these artificial platforms, which were connected by narrow staircases. It was the king's duty to look after this construction, apparently. During heavy rainfall, the fill became heavy and increased the pressure on the walls, thus requiring regular maintenance.
(The ESV Study BibleTM, English Standard Version (ESV) by Crossway Bibles, 2007. Page 551)
King David takes the stronghold of Zion
After David was anointed king over all Israel, King David and his men went to Jerusalem against the Jebusite inhabitants of the land who said to David, "You will not come in here, but the blind and the lame will ward you off"--thinking, "David cannot come in here." David said, "Whoever would strike the Jebusites, let him get up the water shaft to attack 'the lame and the blind,' who are hated by David's soul," and defeated them. Therefore it is said, "The blind and the lame shall not come into the house." David lived in the stronghold and called it the city of David. And David built the city all around from the Millo inward. And David became greater and greater for the LORD, the God of hosts, was with him. (2 Samuel 5:1-10)