What does 1 Chronicles 21:14 mean?

So Gad came to David, and said to him, “Yahweh says, ‘Take your choice:

either three years of famine; or three months to be consumed before your foes, while the sword of your enemies overtakes you; or else three days the sword of Yahweh, even pestilence in the land, and Yahweh’s angel destroying throughout all the borders of Israel. Now therefore consider what answer I shall return to him who sent me.’”

David said to Gad, “I am in distress. Let me fall, I pray, into Yahweh’s hand; for his mercies are very great. Don’t let me fall into man’s hand.”

God sent an angel to Jerusalem to destroy it. As he was about to destroy, Yahweh saw, and he relented of the disaster, and said to the destroying angel, “It is enough. Now withdraw your hand.” Yahweh’s angel was standing by the threshing floor of Ornan the Jebusite.

David lifted up his eyes, and saw Yahweh’s angel standing between earth and the sky, having a drawn sword in his hand stretched out over Jerusalem. Then David and the elders, clothed in sackcloth, fell on their faces.

David said to God, “Isn’t it I who commanded the people to be counted? It is even I who have sinned and done very wickedly; but these sheep, what have they done? Please let your hand, O Yahweh my God, be against me, and against my father’s house; but not against your people, that they should be plagued.”