Now, Isaiah (still channeling God) goes on to offer more prophetic words about Judah and Jerusalem. He says that, in the days to come, God's house will be established on a mountain higher than all other mountains (maybe metaphorically higher).
All the nations, all the Gentiles, and everybody will be coming to gather at this house of God
Everyone is going to dedicate themselves to walking in the ways of Jacob's God—both Jews and Gentiles.
God will maintain order between all the nations so that peace will come everywhere. People will "beat their swords into ploughshares." (In other words, they'll turn weapons of war into farming implements—which is nice.)
Isaiah then addresses Jews themselves, telling the "House of Jacob" to return to following God's ways.
He tells them that they've been too influenced by foreign diviners and soothsayers like the Philistines. Evidently, they've been overly impressed by the riches and splendors of those foreign lands, tempted towards idol-worship.
But the day of the Lord will come, when God will stand taller than all the tall things associated with these other people—like the cedars of Lebanon, the oaks of Bashan, and the ships of Tarshish (in addition to generally tall things like mountains).
God alone will be exalted on that day; all the idols are going to bite the dust. The people are going to chuck their idols into caves and holes, and run away themselves to hide in caves and holes. (Those are going to be some crowded hiding spots.)
The chapter ends with a final exhortation to turn away from everything that is mortal and for the House of Jacob to dedicate itself to God.