This verse sets the stage for a pivotal moment in the conflict between the kingdoms of Judah and Israel. Let's break it down:

"Abijah stood up on Mount Zemaraim, which is in the hill country of Ephraim..." This tells us:
Abijah: He was the king of Judah, the southern kingdom.
Mount Zemaraim: A strategically located mountain within the territory of Ephraim, which belonged to the northern kingdom of Israel. By standing here, Abijah was symbolically and perhaps literally challenging the authority of Jeroboam and the northern kingdom on their own turf. Hill country suggests a position of higher ground, potentially offering a visual and audial advantage for his speech.
Hill Country of Ephraim: This region was a core part of the northern kingdom and held significant historical importance, as it was one of the major tribes of Israel.

"...and said, “Hear me, Jeroboam and all Israel:" This indicates:
Jeroboam: The king of Israel, the northern kingdom, which had seceded from Judah after the reign of Solomon. Jeroboam was a key figure in establishing the northern kingdom and its separate religious practices.
All Israel: Abijah is addressing not just Jeroboam but the entire populace of the northern kingdom. He wants to reach the people with his message.
“Hear me...": This is a direct call for attention. Abijah intends to deliver an important message, likely an argument or a challenge.

In summary, the verse depicts King Abijah of Judah taking a bold stand on enemy territory, Mount Zemaraim, to address King Jeroboam and the entire nation of Israel. It's a dramatic setup for a confrontation, suggesting that Abijah is about to make a significant declaration or argument. The verse is a setup for the speech that follows, where Abijah will argue for the legitimacy of Judah's kingship and religious practices. The location emphasizes the audacity of Abijah's actions and the high stakes of the conflict. He's going behind enemy lines, so to speak, to make his case directly to the people.