What does 1 Chronicles 15:4 mean?

David made himself houses in David’s city; and he prepared a place for God’s ark, and pitched a tent for it.

Then David said, “No one ought to carry God’s ark but the Levites. For Yahweh has chosen them to carry God’s ark, and to minister to him forever.”

David assembled all Israel at Jerusalem, to bring up Yahweh’s ark to its place, which he had prepared for it.

of the sons of Kohath, Uriel the chief, and his brothers one hundred twenty;

of the sons of Merari, Asaiah the chief, and his brothers two hundred twenty;

of the sons of Gershom, Joel the chief, and his brothers one hundred thirty;