This idiom is used to announce that God gave messages to him. Alternate translation: "Yahweh gave messages to him" or "Yahweh spoke to Jeremiah" (See: [[rc://en/ta/man/jit/figs-idiom]])
This is the name of God that he revealed to his people in the Old Testament. See the translationWord page about Yahweh concerning how to translate this.
# in the days of Josiah son of Amon, king of Judah
Here "in the days of" is an idiom that refers to the time when a king reigned. Alternate translation: "when Josiah son of Amon was king of Judah" (See: [[rc://en/ta/man/jit/figs-idiom]])