What does it mean that the kingdom is established in the top of the mountains?
In Old Testament symbolism, a mountain represents a kingdom. Being established in the top of the mountains means God's kingdom, the royal line from David, will become the highest kingdom of kingdoms, exalted above smaller kingdoms, and all nations will flow into it in the last days.
Les explains that in Old Testament symbolism a mountain stands for a kingdom. So when Isaiah says the mountain of the Lord's house will be established in the top of the mountains, it means the kingdom of the Lord's house—the royal line coming from David—will be established as the top kingdom of the kingdoms. The hills represent smaller kingdoms, and this kingdom will be exalted above them. All nations shall flow into it. Once Israel's Messiah comes and sets up His throne and kingdom in Jerusalem, the nations of the world will be drawn to Jerusalem like a magnet drawing steel.
And it shall come to pass in the last days, that the mountain of the LORD’s house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it.