How can it be quiet when the LORD has given it a charge? Against Ashkelon and against the seashore he has appointed it.
This is a brief but violent description of the end God had appointed for one of Israel's historic enemies, Philistia. This was the birthplace of Goliath and of the wicked queen Jezebel. It was a source of constant temptation with Baal worship. Israelites seemed to always envy Philistine trading success and commercial prosperity. But God would make their destruction complete at the hands of the Babylonians.
The ESV Study Bible has this summary:
Philistia was one of Israel's most ancient foes (Josh. 13:2–3; Judg. 3:31; 13:1). This brief chapter asserts that God will destroy the Philistines (Jer. 47:1–4) at the hands of a foe from the north, for his sword cannot rest until then (vv. 5–7). Evidence of such a conflagration appears in the excavations of Ashkelon. Remains from the destruction of the city by Nebuchadnezzar in 604 b. c. include a layer with much smashed pottery and a male skeleton with a crushed skull.
So the Word of God from Jeremiah's day has been visibly confirmed by archeologists today. What God decrees happens. It is that simple. And that powerful.