The “mountain of the LORD’s house” refers to the temple. In Isaiah 2:2–4, Isaiah reported a vision and made a prophecy that has been fulfilled in many ways. He said that when the time comes that the Lord’s people put Him and His house above all things, and when they actively seek counsel from Him by going to His house, then Zion will be established among them and they will have peace and the promise of eternal life.
Another fulfillment of this prophecy has to do with the Lord’s house being established at the “top of the mountains” (Isaiah 2:2). The prophecy has a symbolic reference to putting the temple in the highest place in our lives.
Although it would be natural for one to imagine that Isaiah would think of the physical temple in Jerusalem, it should be kept in mind that the prophets did not always understand the things they prophesied (1 Peter 1:10 & 11).