This verse simply states that King Josiah of Judah observed the Passover festival in Jerusalem, and that the Passover lambs were sacrificed on the 14th day of the first month (Nisan). Let's break it down:

"Josiah kept a Passover to Yahweh in Jerusalem." This establishes that King Josiah, a righteous king who reformed Judah's religious practices, celebrated the Passover. The celebration took place in Jerusalem, which was the designated place for temple worship according to Deuteronomy.

"They killed the Passover on the fourteenth day of the first month." This refers to the specific day the Passover lambs were slaughtered, which is a crucial part of the Passover observance. The "fourteenth day of the first month" refers to the 14th of Nisan in the Hebrew calendar.

Significance:

While the verse itself is straightforward, it's important to understand the context:

Reformation: This Passover was a significant event because it was part of Josiah's wider religious reforms. He had rediscovered the Book of the Law (likely Deuteronomy) and was working to restore proper worship of Yahweh.
Centralized Worship: By holding the Passover in Jerusalem, Josiah was enforcing the Deuteronomic law that required all sacrifices to be offered at the central sanctuary. This was to prevent idolatry and ensure proper observance of the covenant.
Historical Context: The Passover was a national festival that commemorated the Exodus from Egypt, a foundational event in Israel's history. Observing it properly signified covenant faithfulness and remembrance of God's deliverance.
Later Significance: In the New Testament, the Passover took on new meaning as it was during a Passover celebration that Jesus was crucified. Christians see Jesus as the ultimate Passover Lamb who takes away the sins of the world.

In essence, this verse highlights a key moment in Josiah's reign: a return to faithful observance of the Passover, marking a restoration of true worship and covenant faithfulness in Judah.