This verse expresses the fear of Jeroboam, the first king of the northern kingdom of Israel after the kingdom split. Here's a breakdown of what it means:

Context: Jeroboam led a rebellion against Rehoboam, the king of Judah (the southern kingdom). This rebellion resulted in the division of the unified kingdom of Israel into two: Israel (the north) and Judah (the south). Jeroboam feared that his newly established kingdom was unstable.

"If this people goes up to offer sacrifices in Yahweh's house at Jerusalem..." Jerusalem, the capital of Judah, was the location of the Temple, the primary place for worshiping Yahweh (God). Jeroboam understood that the religious practices of his people were still centered in the south.

"...then the heart of this people will turn again to their lord, even to Rehoboam king of Judah..." This is Jeroboam's main fear. He believed that if the people of the northern kingdom continued to travel to Jerusalem to worship, they would feel a renewed sense of loyalty to Rehoboam, their former king. The act of worshiping in Jerusalem would remind them of their shared history and heritage.

"...and they will kill me, and return to Rehoboam king of Judah." This is the ultimate consequence Jeroboam feared. He believed that the people, influenced by their religious connection to Jerusalem and Rehoboam, would turn against him, assassinate him, and reunite the kingdom under Rehoboam.

In essence, this verse shows Jeroboam's concern that the religious practices of his people would undermine his political authority. He feared that religious unity in Jerusalem would lead to political reunification under Rehoboam, costing him his power and potentially his life. This fear is what motivates Jeroboam to establish alternative worship sites and practices in the northern kingdom, a decision that ultimately leads to religious schism and is condemned in the biblical narrative.